gdb/riscv: use register name enum values in riscv-linux-nat.c

There were a few places where we were using integer values rather than
the RISCV_*_REGNUM constants defined in riscv-tdep.h.  This commit
replaces 0 with RISCV_ZERO_REGNUM and 32 with RISCV_PC_REGNUM in a few
places.

There should be no user visible changes after this commit.