[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.

top/include/lk/compiler.h

@@ -11,6 +11,9 @@
 
 #pragma once
 
+#define __CONCAT1(x, y) x ## y
+#define __CONCAT(x, y) __CONCAT1(x, y)
+
 #ifndef __ASSEMBLY__
 
 #if __GNUC__ || __clang__