RISC-V: Don't emit 2-byte NOPs if the C extension is disabled

RISC-V: Don't emit 2-byte NOPs if the C extension is disabled

Systems without the C extension mandate 4-byte alignment for
instructions, so there is no reason to allow for 2-byte alignment.  This
change avoids emitting lots of unimplemented instructions into object
files on non-C targets, which users keep reporting as a bug.  While this
isn't actually a bug (as none of the offsets in object files are
relevant until RISC-V), it is ugly.

gas/ChangeLog

2017-10-23  Palmer Dabbelt  <palmer@dabbelt.com>

        * config/tc-riscv.c (riscv_frag_align_code): Align code by 4
        bytes on non-RVC systems.