all: add the Boehm-Demers-Weiser GC on Linux

This adds support for the well-known Boehm GC. It's significantly faster
than our own naive GC and could be used as an alternative on bigger
systems.

In the future, this GC might also be supported on WebAssembly with some
extra work. Right now it's Linux only (though Windows/MacOS shouldn't be
too difficult to add).

compileopts/target.go

@@ -247,7 +247,6 @@ func defaultTarget(options *Options) (*TargetSpec, error) {
 		GOOS:             options.GOOS,
 		GOARCH:           options.GOARCH,
 		BuildTags:        []string{options.GOOS, options.GOARCH},
-		GC:               "precise",
 		Scheduler:        "tasks",
 		Linker:           "cc",
 		DefaultStackSize: 1024 * 64, // 64kB
@@ -366,6 +365,7 @@ func defaultTarget(options *Options) (*TargetSpec, error) {
 	llvmvendor := "unknown"
 	switch options.GOOS {
 	case "darwin":
+		spec.GC = "precise"
 		platformVersion := "10.12.0"
 		if options.GOARCH == "arm64" {
 			platformVersion = "11.0.0" // first macosx platform with arm64 support
@@ -388,6 +388,7 @@ func defaultTarget(options *Options) (*TargetSpec, error) {
 			"src/runtime/runtime_unix.c",
 			"src/runtime/signal.c")
 	case "linux":
+		spec.GC = "boehm"
 		spec.Linker = "ld.lld"
 		spec.RTLib = "compiler-rt"
 		spec.Libc = "musl"
@@ -411,6 +412,7 @@ func defaultTarget(options *Options) (*TargetSpec, error) {
 			"src/runtime/runtime_unix.c",
 			"src/runtime/signal.c")
 	case "windows":
+		spec.GC = "precise"
 		spec.Linker = "ld.lld"
 		spec.Libc = "mingw-w64"
 		// Note: using a medium code model, low image base and no ASLR