feat(asm): the operand forms and defines GOROOT writes

Assisted-by: GLM 5.3 Flash

parser/preproc.go

@@ -34,6 +34,12 @@ type Options struct {
 	// Expand enables macro expansion, include splicing and the
 	// statement-separator reading of ';' that the expanded bodies rely on.
 	Expand bool
+	// Predefines names the macros defined before the file is read.  The
+	// go command drives go tool asm with -D GOOS_<goos> -D GOARCH_<arch>,
+	// and GOROOT's own headers (go_tls.h, asm_riscv64.h) select their
+	// platform blocks with #ifdef on exactly those names, so an assembler
+	// without them cannot see the platform definitions at all.
+	Predefines map[string]string
 }
 
 // ParseWithOptions parses src like Parse, optionally preprocessing it first.
@@ -44,6 +50,9 @@ func ParseWithOptions(path, src string, opts Options) (*ast.File, []error) {
 	var errs []error
 	if opts.Expand {
 		pp := &preproc{opts: opts, macros: map[string]*macroDef{}}
+		for name, value := range opts.Predefines {
+			pp.macros[name] = &macroDef{name: name, body: lexer.Tokenize(value)}
+		}
 		lines = pp.fileLines(path, tokens, token.Position{})
 		errs = pp.errs
 	} else {