all: improve compiler-rt compilation

A few changes to make sure compiler-rt is correctly compiled (and
doesn't include host headers, for example).

This improves support for AVR, but it still doesn't work. Compiler-rt
itself doesn't really work for AVR either.

builtins.go

@@ -161,7 +161,7 @@ func loadBuiltins(target string) (path string, err error) {
 		objpath := filepath.Join(dir, name+".o")
 		objs = append(objs, objpath)
 		srcpath := filepath.Join(builtinsDir, name+".c")
-		cmd := exec.Command(commands["clang"], "-c", "-Oz", "-g", "-Werror", "-Wall", "-std=c11", "--target="+target, "-o", objpath, srcpath)
+		cmd := exec.Command(commands["clang"], "-c", "-Oz", "-g", "-Werror", "-Wall", "-std=c11", "-fshort-enums", "-nostdlibinc", "--target="+target, "-o", objpath, srcpath)
 		cmd.Stdout = os.Stdout
 		cmd.Stderr = os.Stderr
 		cmd.Dir = dir

src/runtime/runtime_arm.go

@@ -53,3 +53,11 @@ func abort() {
 func align(ptr uintptr) uintptr {
 	return (ptr + 3) &^ 3
 }
+
+// Implement memset for compiler-rt.
+//go:export memset
+func memset(ptr unsafe.Pointer, c byte, size uintptr) {
+	for i := uintptr(0); i < size; i++ {
+		*(*byte)(unsafe.Pointer(uintptr(ptr) + i)) = c
+	}
+}

targets/microbit.json

@@ -12,7 +12,6 @@
 		"-Wl,--gc-sections",
 		"-fno-exceptions", "-fno-unwind-tables",
 		"-ffunction-sections", "-fdata-sections",
-		"-fno-short-enums",
 		"-Os",
 		"-DNRF51",
 		"-Ilib/CMSIS/CMSIS/Include",