RISC-V: Inline the assembly register save/restore macros

These are only used once, and when reading the code I've always found them to
be more of a headache than a benefit.  While they were never worth removing
before, LLVM's integrated assembler doesn't support LOCAL so rather that trying
to figure out how to refactor the macros it seems saner to just inline them.

Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>