RISC-V: Add SiFive extensions, xsfvcp and xsfcease

RISC-V: Add SiFive extensions, xsfvcp and xsfcease

We have already upstreamed these extensions into binutils, and now we need GCC
to recognize these extensions and pass them to binutils as well. We also plan
to upstream intrinsics in the near future. :)

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc (riscv_implied_info): Add xsfvcp.
	(riscv_ext_version_table): Add xsfvcp, xsfcease.
	(riscv_ext_flag_table): Ditto.
	* config/riscv/riscv.opt (riscv_sifive_subext): New.
	(XSFVCP): New.
	(XSFCEASE): New.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/predef-sf-1.c: New.
	* gcc.target/riscv/predef-sf-2.c: New.