wasm: add Boehm GC support

This adds support for `-gc=boehm` on `-target=wasip1` and `-target=wasm`
(in a browser or NodeJS). Notably it does *not* add Boehm GC support for
`-target=wasip2`, since that target doesn't have a real libc.

builder/mingw-w64.go

@@ -48,7 +48,7 @@ var libMinGW = Library{
 		}
 		return flags
 	},
-	librarySources: func(target string) ([]string, error) {
+	librarySources: func(target string, _ bool) ([]string, error) {
 		// These files are needed so that printf and the like are supported.
 		var sources []string
 		if strings.Split(target, "-")[0] == "i386" {