[RISCV] Allow fp as an alias of s0

The RISC-V Assembly Programmer's Manual defines fp as another alias of x8.
However, our tablegen rules only recognise s0. This patch adds fp as another
alias of x8. GCC also accepts fp.

Differential Revision: https://reviews.llvm.org/D59209
Patch by Ferran Pallarès Roca.

llvm-svn: 355867