compiler: support file-level //go:linkname directives

Modern golang.org/x/sys/unix (v0.36+) declares its linknames
detached from function declarations, e.g.:

    func syscall_syscall(...)
    //go:linkname syscall_syscall syscall.syscall

TinyGo's pragma parser only inspected function doc comments and
therefore missed these, producing link errors like:

    undefined symbol: _golang.org/x/sys/unix.syscall_syscall

Extend parsePragmas to also walk the enclosing *ast.File's
free-standing comments for //go:linkname directives matching the
function's name. Function-attached directives still take
precedence. The existing 'unsafe' import gate is preserved.

Fixes #4395, #5365

compiler/testdata/pragma.go

@@ -120,3 +120,31 @@ func stillEscapes(a *int, b []int, c chan int, d *[0]byte) {
 func doesHeapAlloc() *int {
 	return new(int)
 }
+
+// Define a function in a different package using a file-level go:linkname.
+// (Same as withLinkageName1, but with the //go:linkname directive detached
+// from the function declaration — see https://github.com/tinygo-org/tinygo/issues/4395)
+func withFileLevelLinkageName1() {
+}
+
+// Import a function from a different package using a file-level go:linkname.
+// (Same as withLinkageName2, but with the //go:linkname directive detached
+// from the function declaration.)
+func withFileLevelLinkageName2()
+
+//go:linkname withFileLevelLinkageName1 somepkg.someFileLevelFunction1
+//go:linkname withFileLevelLinkageName2 somepkg.someFileLevelFunction2
+
+// File-level linkname directives can also appear between two function
+// declarations, in which case Go's AST attaches them as the doc comment
+// of the following function — even when the directive's localname refers
+// to a different function. Exercise that case: the directive below names
+// withAdjacentLinkageName, but Go will attach it to
+// sentinelAfterAdjacentLinkname's Doc. The file-level scan must find it
+// by walking comment groups regardless of which decl they're attached to.
+func withAdjacentLinkageName() {
+}
+
+//go:linkname withAdjacentLinkageName somepkg.someAdjacentFunction
+func sentinelAfterAdjacentLinkname() {
+}

compiler/testdata/pragma.ll

@@ -102,6 +102,26 @@ entry:
 ; Function Attrs: allockind("alloc,zeroed") allocsize(0)
 declare noalias nonnull ptr @runtime.alloc_noheap(i32, ptr, ptr) #9
 
+; Function Attrs: nounwind
+define hidden void @somepkg.someFileLevelFunction1(ptr %context) unnamed_addr #1 {
+entry:
+  ret void
+}
+
+declare void @somepkg.someFileLevelFunction2(ptr) #0
+
+; Function Attrs: nounwind
+define hidden void @somepkg.someAdjacentFunction(ptr %context) unnamed_addr #1 {
+entry:
+  ret void
+}
+
+; Function Attrs: nounwind
+define hidden void @main.sentinelAfterAdjacentLinkname(ptr %context) unnamed_addr #1 {
+entry:
+  ret void
+}
+
 attributes #0 = { "target-features"="+bulk-memory,+bulk-memory-opt,+call-indirect-overlong,+mutable-globals,+nontrapping-fptoint,+sign-ext,-multivalue,-reference-types" }
 attributes #1 = { nounwind "target-features"="+bulk-memory,+bulk-memory-opt,+call-indirect-overlong,+mutable-globals,+nontrapping-fptoint,+sign-ext,-multivalue,-reference-types" }
 attributes #2 = { nounwind "target-features"="+bulk-memory,+bulk-memory-opt,+call-indirect-overlong,+mutable-globals,+nontrapping-fptoint,+sign-ext,-multivalue,-reference-types" "wasm-export-name"="extern_func" }