[riscv][clang] Use a CSR name instead of a numeric expression

Clang's assembler rejects expressions containing e.g. (1u << N) in the
assembler. Instead using numeric expressions for per-privilege level
CSRs, we can prepend `m` or `s`. This also lets the compiler assign the
CSR encoding instead of having to hardcode it in the source code.

platform/rp20xx/include/sys/cdefs.h

@@ -1,6 +0,0 @@
-#pragma once
-
-#ifndef __CONCAT
-#define __CONCAT1(x,y) x ## y
-#define __CONCAT(x,y) __CONCAT1(x,y)
-#endif