[vm / library] Foreign function interface prototype

Prototype for `dart:ffi` on Linux/MacOS x64 in JIT mode.
`dart:ffi` is experimental and its API is likely to change in the future.
Progress and design decisions are tracked in https://github.com/dart-lang/sdk/projects/13


issue: dart-lang#34452
Change-Id: Ifa4566388e42c8757f154741d11e303465ef305d
Cq-Include-Trybots: luci.dart.try:vm-kernel-optcounter-threshold-linux-release-x64-try, vm-kernel-precomp-linux-debug-x64-try, vm-kernel-precomp-linux-release-simarm-try, vm-kernel-precomp-linux-release-simarm64-try, vm-kernel-precomp-linux-release-x64-try, vm-kernel-precomp-mac-release-simarm64-try, vm-kernel-precomp-win-release-x64-try, vm-kernel-mac-debug-x64-try, vm-kernel-asan-linux-release-x64
Reviewed-on: https://dart-review.googlesource.com/c/80124
Reviewed-by: Samir Jindel <sjindel@google.com>
Auto-Submit: Daco Harkes <dacoharkes@google.com>

assembler_x64.h

@@ -474,6 +474,10 @@ class Assembler : public AssemblerBase {
   // for proper unwinding of Dart frames (use --generate_gdb_symbols and -O0).
   void movq(Register dst, Register src) { EmitQ(src, dst, 0x89); }
 
+  void movq(XmmRegister dst, Register src) {
+    EmitQ(dst, src, 0x6E, 0x0F, 0x66);
+  }
+
   void movd(XmmRegister dst, Register src) {
     EmitL(dst, src, 0x6E, 0x0F, 0x66);
   }