compiler: add //go:linkname support for globals

We previously used our own //go:extern but //go:linkname is probably the
better choice since it's used in the math/bits package.

compiler/testdata/pragma.go

@@ -2,6 +2,12 @@ package main
 
 import _ "unsafe"
 
+// Use the go:linkname mechanism to link this global to a different package.
+// This is used in math/bits.
+//
+//go:linkname linknamedGlobal runtime.testLinknamedGlobal
+var linknamedGlobal int
+
 // Creates an external global with name extern_global.
 //
 //go:extern extern_global

compiler/testdata/pragma.ll

@@ -3,6 +3,7 @@ source_filename = "pragma.go"
 target datalayout = "e-m:e-p:32:32-p10:8:8-p20:8:8-i64:64-n32:64-S128-ni:1:10:20"
 target triple = "wasm32-unknown-wasi"
 
+@runtime.testLinknamedGlobal = external global i32, align 4
 @extern_global = external global [0 x i8], align 1
 @main.alignedGlobal = hidden global [4 x i32] zeroinitializer, align 32
 @main.alignedGlobal16 = hidden global [4 x i32] zeroinitializer, align 16