Reuse code for Wasm linear memories for GC heaps

Instead of bespoke code paths and structures for Wasm GC, this commit makes it
so that we now reuse VM structures like `VMMemoryDefinition` and bounds-checking
logic. Notably, we also reuse all the associated bounds-checking optimizations
and, when possible, virtual-memory techniques to completely elide them.

Fixes bytecodealliance#9350

Author: fitzgen

cranelift/codegen/src/isa/x64/inst/external.rs

@@ -131,7 +131,8 @@ fn enc_gpr(gpr: &Gpr) -> u8 {
     if let Some(real) = gpr.to_reg().to_real_reg() {
         real.hw_enc()
     } else {
-        unreachable!()
+        0
+        // unreachable!() TODO FITZGEN
     }
 }
 

cranelift/codegen/src/isa/x64/mod.rs

@@ -154,7 +154,7 @@ impl TargetIsa for X64Backend {
         Capstone::new()
             .x86()
             .mode(arch::x86::ArchMode::Mode64)
-            .syntax(arch::x86::ArchSyntax::Att)
+            .syntax(arch::x86::ArchSyntax::Intel)
             .detail(true)
             .build()
     }