Address review comments:

- Undo temporary changes to default features (`all-arch`) and a
  signal-handler test.
- Remove `SIGTRAP` handler: no longer needed now that we've found an
  "undefined opcode" option on ARM64.
- Rename pp.rs to pretty_print.rs in machinst/.
- Only use empty stack-probe on non-x86. As per a comment in
  rust-lang/compiler-builtins [1], LLVM only supports stack probes on
  x86 and x86-64. Thus, on any other CPU architecture, we cannot refer
  to `__rust_probestack`, because it does not exist.
- Rename arm64 to aarch64.
- Use `target` directive in vcode filetests.
- Run the flags verifier, but without encinfo, when using new backends.
- Clean up warning overrides.
- Fix up use of casts: use u32::from(x) and siblings when possible,
  u32::try_from(x).unwrap() when not, to avoid silent truncation.
- Take immutable `Function` borrows as input; we don't actually
  mutate the input IR.
- Lots of other miscellaneous cleanups.

[1] cae3e6ea23/src/probestack.rs (L39)

cranelift/filetests/filetests/vcode/aarch64/arithmetic.clif

@@ -1,4 +1,5 @@
-test vcode arch=arm64
+test vcode
+target aarch64
 
 function %f(i64, i64) -> i64 {
 block0(v0: i64, v1: i64):

cranelift/filetests/filetests/vcode/aarch64/basic1.clif

@@ -1,4 +1,5 @@
-test vcode arch=arm64
+test vcode
+target aarch64
 
 function %f(i32, i32) -> i32 {
 block0(v0: i32, v1: i32):

cranelift/filetests/filetests/vcode/aarch64/bitops.clif

@@ -1,4 +1,5 @@
-test vcode arch=arm64
+test vcode
+target aarch64
 
 function %a(i32) -> i32 {
 block0(v0: i32):

cranelift/filetests/filetests/vcode/aarch64/call-indirect.clif

@@ -1,4 +1,5 @@
-test vcode arch=arm64
+test vcode
+target aarch64
 
 function %f(i64, i64) -> i64 {
     sig0 = (i64) -> i64

cranelift/filetests/filetests/vcode/aarch64/call.clif

@@ -1,4 +1,5 @@
-test vcode arch=arm64
+test vcode
+target aarch64
 
 function %f(i64) -> i64 {
     fn0 = %g(i64) -> i64

cranelift/filetests/filetests/vcode/aarch64/condbr.clif

@@ -1,4 +1,5 @@
-test vcode arch=arm64
+test vcode
+target aarch64
 
 function %f(i64, i64) -> b1 {
 block0(v0: i64, v1: i64):
@@ -33,7 +34,7 @@ block2:
 ; nextln: mov fp, sp
 ; nextln: subs xzr, x0, x1
 ; nextln: b.eq 20
-; check: Block 0:
+; check: Block 2:
 ; check: movz x0, #2
 ; nextln: mov sp, fp
 ; nextln: ldp fp, lr, [sp], #16
@@ -58,7 +59,7 @@ block1:
 ; check: stp fp, lr, [sp, #-16]!
 ; nextln: mov fp, sp
 ; nextln: subs xzr, x0, x1
-; check: Block 0:
+; check: Block 1:
 ; check: movz x0, #1
 ; nextln: mov sp, fp
 ; nextln: ldp fp, lr, [sp], #16

cranelift/filetests/filetests/vcode/aarch64/condops.clif

@@ -1,4 +1,5 @@
-test vcode arch=arm64
+test vcode
+target aarch64
 
 function %f(i8, i64, i64) -> i64 {
 block0(v0: i8, v1: i64, v2: i64):

cranelift/filetests/filetests/vcode/aarch64/constants.clif

@@ -1,4 +1,5 @@
-test vcode arch=arm64
+test vcode
+target aarch64
 
 function %f() -> i64 {
 block0:

cranelift/filetests/filetests/vcode/aarch64/extend-op.clif

@@ -1,4 +1,5 @@
-test vcode arch=arm64
+test vcode
+target aarch64
 
 function %f(i8) -> i64 {
 block0(v0: i8):

cranelift/filetests/filetests/vcode/aarch64/jumptable.clif

@@ -1,4 +1,5 @@
-test vcode arch=arm64
+test vcode
+target aarch64
 
 function %f(i64) -> i64 {
   jt0 = jump_table [block1, block2, block3]

cranelift/filetests/filetests/vcode/aarch64/narrow-arithmetic.clif

@@ -1,4 +1,5 @@
-test vcode arch=arm64
+test vcode
+target aarch64
 
 function %add8(i8, i8) -> i8 {
 block0(v0: i8, v1: i8):

cranelift/filetests/filetests/vcode/aarch64/saturating-ops.clif

@@ -1,4 +1,5 @@
-test vcode arch=arm64
+test vcode
+target aarch64
 
 function %uaddsat64(i64, i64) -> i64 {
 block0(v0: i64, v1: i64):

cranelift/filetests/filetests/vcode/aarch64/shift-op.clif

@@ -1,4 +1,5 @@
-test vcode arch=arm64
+test vcode
+target aarch64
 
 function %f(i64) -> i64 {
 block0(v0: i64):

cranelift/filetests/filetests/vcode/aarch64/shift-rotate.clif

@@ -1,4 +1,5 @@
-test vcode arch=arm64
+test vcode
+target aarch64
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; ROR, variable

cranelift/filetests/filetests/vcode/aarch64/symbol-value.clif

@@ -1,4 +1,5 @@
-test vcode arch=arm64
+test vcode
+target aarch64
 
 function %f() -> i64 {
   gv0 = symbol %my_global

cranelift/filetests/filetests/vcode/aarch64/traps.clif

@@ -1,4 +1,5 @@
-test vcode arch=arm64
+test vcode
+target aarch64
 
 function %f() {
 block0:

cranelift/filetests/filetests/vcode/aarch64/uextend-sextend.clif

@@ -1,4 +1,5 @@
-test vcode arch=arm64
+test vcode
+target aarch64
 
 function %f_u_8_64(i8) -> i64 {
 block0(v0: i8):