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/compiler.go

@@ -90,7 +90,8 @@ type compilerContext struct {
 	astComments      map[string]*ast.CommentGroup
 	embedGlobals     map[string][]*loader.EmbedFile
 	pkg              *types.Package
-	packageDir       string // directory for this package
+	loaderPkg        *loader.Package // current package being compiled (for AST access)
+	packageDir       string          // directory for this package
 	runtimePkg       *types.Package
 	localTypeNames   typeutil.Map // *types.Named (synthetic local from generic instantiation) -> string
 }
@@ -298,6 +299,7 @@ func CompilePackage(moduleName string, pkg *loader.Package, ssaPkg *ssa.Package,
 	c.packageDir = pkg.OriginalDir()
 	c.embedGlobals = pkg.EmbedGlobals
 	c.pkg = pkg.Pkg
+	c.loaderPkg = pkg
 	c.runtimePkg = ssaPkg.Prog.ImportedPackage("runtime").Pkg
 	c.program = ssaPkg.Prog