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/wasilibc.go

@@ -120,7 +120,7 @@ var libWasiLibc = Library{
 		return nil
 	},
 	sourceDir: func() string { return filepath.Join(goenv.Get("TINYGOROOT"), "lib/wasi-libc") },
-	librarySources: func(target string) ([]string, error) {
+	librarySources: func(target string, libcNeedsMalloc bool) ([]string, error) {
 		type filePattern struct {
 			glob    string
 			exclude []string
@@ -169,6 +169,11 @@ var libWasiLibc = Library{
 			{glob: "libc-bottom-half/sources/*.c"},
 		}
 
+		// We're using the Boehm GC, so we need a heap implementation in the libc.
+		if libcNeedsMalloc {
+			globs = append(globs, filePattern{glob: "dlmalloc/src/dlmalloc.c"})
+		}
+
 		// See: LIBC_TOP_HALF_MUSL_SOURCES in the Makefile
 		sources := []string{
 			"libc-top-half/musl/src/misc/a64l.c",