RISC-V: Allow ISA subsets to be disabled

RISC-V: Allow ISA subsets to be disabled

Without this patch, passing "-march=rv64ic -march=rv64i" results in
you getting a "RV64IC" toolchain, which isn't expected.

gas/ChangeLog:

2017-03-30  Palmer Dabbelt  <palmer@dabbelt.com>

       * config/tc-riscv.c (riscv_clear_subsets): New function.
       (riscv_add_subset): Call riscv_clear_subsets and riscv_set_rvc to
       clear RVC when it's been previously set.