Use global_asm! instead of external assembly files (#4306)

* Use `global_asm!` instead of external assembly files

This commit moves the external assembly files of the `wasmtime-fiber`
crate into `global_asm!` blocks defined in Rust. The motivation for
doing this is not very strong at this time, but the points in favor of
this are:

* One less tool needed to cross-compile Wasmtime. A linker is still
  needed but perhaps one day that will improve as well.
* A "modern" assembler, built-in to LLVM, is used instead of whatever
  appears on the system.

The first point hasn't really cropped up that much and typically getting
an assembler is just as hard as getting a linker nowadays. The second
point though has us using `hint #xx` in aarch64 assembly instead of the
actual instructions for assembler compatibility, and I believe that's no
longer necessary because the LLVM assembler supports the modern
instruction names.

The translation of the x86/x86_64 assembly has been done to Intel
syntax as well as opposed to the old AT&T syntax since that's Rust's
default. Additionally s390x still remains in an external assembler file
because `global_asm!` is still unstable in Rust on that platform.

* Simplify alignment specification

* Temporarily disable fail-fast

* Add `.cfi_def_cfa_offset 0` to fix CI

* Turn off fail-fast

* Review comments

crates/fiber/src/unix/arm.rs

@@ -0,0 +1,83 @@
+// A WORD OF CAUTION
+//
+// This entire file basically needs to be kept in sync with itself. It's not
+// really possible to modify just one bit of this file without understanding
+// all the other bits. Documentation tries to reference various bits here and
+// there but try to make sure to read over everything before tweaking things!
+//
+// Also at this time this file is heavily based off the x86_64 file, so you'll
+// probably want to read that one as well.
+
+// fn(top_of_stack(%r0): *mut u8)
+asm_func!(
+    "wasmtime_fiber_switch",
+    "
+        // Save callee-saved registers
+        push {{r4-r11,lr}}
+
+        // Swap stacks, recording our current stack pointer
+        ldr r4, [r0, #-0x08]
+        str sp, [r0, #-0x08]
+        mov sp, r4
+
+        // Restore and return
+        pop {{r4-r11,lr}}
+        bx lr
+    ",
+);
+
+// fn(
+//    top_of_stack(%r0): *mut u8,
+//    entry_point(%r1): extern fn(*mut u8, *mut u8),
+//    entry_arg0(%r2): *mut u8,
+// )
+asm_func!(
+    "wasmtime_fiber_init",
+    "
+        adr r3, wasmtime_fiber_start
+        str r3, [r0, #-0x0c] // => lr
+        str r0, [r0, #-0x10] // => r11
+        str r1, [r0, #-0x14] // => r10
+        str r2, [r0, #-0x18] // => r9
+
+        add r3, r0, #-0x2c
+        str r3, [r0, #-0x08]
+        bx lr
+    ",
+);
+
+asm_func!(
+    "wasmtime_fiber_start",
+    "
+        .cfi_startproc simple
+        .cfi_def_cfa_offset 0
+        // See the x86_64 file for more commentary on what these CFI directives
+        // are doing. Like over there note that the relative offsets to
+        // registers here match the frame layout in `wasmtime_fiber_switch`.
+        //
+        // TODO: this is only lightly tested. This gets backtraces in gdb but
+        // not at runtime. Perhaps the libgcc at runtime was too old? Doesn't
+        // support something here? Unclear. Will need investigation if someone
+        // ends up needing this and it still doesn't work.
+        .cfi_escape 0x0f,    /* DW_CFA_def_cfa_expression */ \
+            5,               /* the byte length of this expression */ \
+            0x7d, 0x00,      /* DW_OP_breg14(%sp) + 0 */ \
+            0x06,            /* DW_OP_deref */ \
+            0x23, 0x24	 /* DW_OP_plus_uconst 0x24 */
+
+        .cfi_rel_offset lr, -0x04
+        .cfi_rel_offset r11, -0x08
+        .cfi_rel_offset r10, -0x0c
+        .cfi_rel_offset r9, -0x10
+        .cfi_rel_offset r8, -0x14
+        .cfi_rel_offset r7, -0x18
+        .cfi_rel_offset r6, -0x1c
+        .cfi_rel_offset r5, -0x20
+        .cfi_rel_offset r4, -0x24
+
+        mov r1, r11
+        mov r0, r9
+        blx r10
+        .cfi_endproc
+    ",
+);