RISC-V: Prohibit the 'Zcf' extension on RV64

RISC-V: Prohibit the 'Zcf' extension on RV64

As per:
<https://github.com/riscv/riscv-code-size-reduction/issues/221>,
the 'Zcf' extension does not exist on RV64.  This is reflected on the
version 1.0.4-1 of the code size reduction specification:
<https://github.com/riscv/riscv-code-size-reduction/releases/tag/v1.0.4-1>.

This commit prohibits the combination: RV64 (or any ISA with XLEN > 32)
and the 'Zcf' extension.

bfd/ChangeLog:

	* elfxx-riscv.c (riscv_parse_check_conflicts): Prohibit
	combination of RV64 and 'Zcf'.

gas/ChangeLog:

	* testsuite/gas/riscv/march-fail-rv64i_zcf.d: New test.
	* testsuite/gas/riscv/march-fail-rv64i_zcf.l: Likewise.