windows: add support for the Boehm-Demers-Weiser GC

A few small changes were needed to make this work. In particular, I
found a critical bug (see the previous commit) that needed to be fixed
to make this work on Windows.

compileopts/config.go

@@ -395,8 +395,10 @@ func (c *Config) LibcCFlags() []string {
 			"-nostdlibinc",
 			"-isystem", filepath.Join(path, "include"),
 			"-isystem", filepath.Join(root, "lib", "mingw-w64", "mingw-w64-headers", "crt"),
+			"-isystem", filepath.Join(root, "lib", "mingw-w64", "mingw-w64-headers", "include"),
 			"-isystem", filepath.Join(root, "lib", "mingw-w64", "mingw-w64-headers", "defaults", "include"),
 			"-D_UCRT",
+			"-D_WIN32_WINNT=0x0a00", // target Windows 10
 		}
 	case "":
 		// No libc specified, nothing to add.

compileopts/target.go

@@ -427,7 +427,7 @@ func defaultTarget(options *Options) (*TargetSpec, error) {
 			"src/runtime/runtime_unix.c",
 			"src/runtime/signal.c")
 	case "windows":
-		spec.GC = "precise"
+		spec.GC = "boehm"
 		spec.Linker = "ld.lld"
 		spec.Libc = "mingw-w64"
 		// Note: using a medium code model, low image base and no ASLR