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.

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
+	}
+}