c99 elfxx-riscv.c fix

We can't use c99 without enabling c99 support for older compilers
that don't enable c99 by default.  So if you want to use c99 contructs
in binutils you'll need to first arrange for -std=c99 to be passed to
older compilers.

	* elfxx-riscv.c (riscv_multi_letter_ext_valid_p): Don't use C99.