Support RISC-V variants both on GCC and Clang

[carlescufi]

From our (Nordic) perspective, we would like the following RV variants to be supported in both toolchains:

• RV32
  • rv32emc_zicsr
  • rv32emc_zicsr_zba_zbb_zbc_zbs_zcb
  • rv32emc_zicsr_zba_zbb_zbc_zbs_zcb_zicntr
• RV64
  • rv64imafdc_zicsr_zifencei_zba_zbb_zbc_zbs

[stephanosio]

Keeping this open until Clang multi-libs are synced with GCC.