Resolve libcall relocations for older CPUs (#5567)

* Resolve libcall relocations for older CPUs

Long ago Wasmtime used to have logic for resolving relocations
post-compilation for libcalls which I ended up removing during
refactorings last year. As #5563 points out, however, it's possible to
get Wasmtime to panic by disabling SSE features which forces Cranelift
to use libcalls for some floating-point operations instead. Note that
this also requires disabling SIMD because SIMD support has a baseline of
SSE 4.2.

This commit pulls back the old implementations of various libcalls and
reimplements logic necessary to have them work on CPUs without SSE 4.2

Closes #5563

* Fix log message in `wast` support

* Fix offset listed in relocations

Be sure to factor in the offset of the function itself

* Review comments

crates/environ/src/obj.rs

@@ -48,7 +48,7 @@ pub const ELF_WASMTIME_ADDRMAP: &str = ".wasmtime.addrmap";
 /// encodes the ability to map an offset in the text section to the trap code
 /// that it corresponds to.
 ///
-/// This section is used at runtime to determine what flavor fo trap happened to
+/// This section is used at runtime to determine what flavor of trap happened to
 /// ensure that embedders and debuggers know the reason for the wasm trap. The
 /// encoding of this section is custom to Wasmtime and managed with helpers in
 /// the `object` crate:
@@ -63,7 +63,7 @@ pub const ELF_WASMTIME_ADDRMAP: &str = ".wasmtime.addrmap";
 ///
 /// This section is decoded by `lookup_trap_code` below which will read the
 /// section count, slice some bytes to get the various arrays, and then perform
-/// a binary search on the offsets array to find the an index corresponding to
+/// a binary search on the offsets array to find the index corresponding to
 /// the pc being looked up. If found the same index in the trap array (the array
 /// of bytes) is the trap code for that offset.
 ///
@@ -128,3 +128,42 @@ pub const ELF_NAME_DATA: &'static str = ".name.wasm";
 /// and is instead indexed directly by relative indices stored in compilation
 /// metadata.
 pub const ELF_WASMTIME_DWARF: &str = ".wasmtime.dwarf";
+
+macro_rules! libcalls {
+    ($($rust:ident = $sym:tt)*) => (
+        #[allow(missing_docs)]
+        pub enum LibCall {
+            $($rust,)*
+        }
+
+        impl LibCall {
+            /// Returns the libcall corresponding to the provided symbol name,
+            /// if one matches.
+            pub fn from_str(s: &str) -> Option<LibCall> {
+                match s {
+                    $($sym => Some(LibCall::$rust),)*
+                    _ => None,
+                }
+            }
+
+            /// Returns the symbol name in object files associated with this
+            /// libcall.
+            pub fn symbol(&self) -> &'static str {
+                match self {
+                    $(LibCall::$rust => $sym,)*
+                }
+            }
+        }
+    )
+}
+
+libcalls! {
+    FloorF32 = "libcall_floor32"
+    FloorF64 = "libcall_floor64"
+    NearestF32 = "libcall_nearestf32"
+    NearestF64 = "libcall_nearestf64"
+    CeilF32 = "libcall_ceilf32"
+    CeilF64 = "libcall_ceilf64"
+    TruncF32 = "libcall_truncf32"
+    TruncF64 = "libcall_truncf64"
+}