[libc] Add support for setjmp and longjmp in riscv

This patch implements setjmp and longjmp in riscv using inline asm. The
following changes were required:

* Omit frame pointer: otherwise gcc won't allow us to use s0
* Use __attribute__((naked)): otherwise both gcc and clang will generate
function prologue and epilogue in both functions. This doesn't happen
in x86_64, so we guard it to only riscv

Furthermore, using __attribute__((naked)) causes two problems: we
can't use `return 0` (both gcc and clang) and the function arguments in
the function body (clang only), so we had to use a0 and a1 directly.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D145584