RISC-V: Implications from 'Zc[fd]' extensions

RISC-V: Implications from 'Zc[fd]' extensions

The version 1.0.4-1 of the code size reduction specification clarifies
that 'Zcf' implies 'F' and 'Zcd' implies 'D'.

cf:
<https://github.com/riscv/riscv-code-size-reduction/releases/tag/v1.0.4-1>

This commit adds those implications.

bfd/ChangeLog:

	* elfxx-riscv.c (riscv_implicit_subsets): Add two implications,
	'Zcf' -> 'F' and 'Zcd' -> 'D'.

gas/ChangeLog:

	* testsuite/gas/riscv/march-imply-zcd.d: New test.
	* testsuite/gas/riscv/march-imply-zcf.d: New test.