libc: riscv: Fix the RISC-V ifdef

Following the standard GCC RISC-V convetion use __riscv for the RISC-V
specific define:
https://github.com/gcc-mirror/gcc/blob/41d6b10e96a1de98e90a7c0378437c3255814b16/gcc/config/riscv/riscv-c.c#L37



Signed-off-by: Alistair Francis <alistair.francis@wdc.com>