RISC-V: Add support for 'Zvfh' and 'Zvfhmin'

RISC-V: Add support for 'Zvfh' and 'Zvfhmin'

This commit adds support for recently ratified vector FP16 extensions:
'Zvfh' and 'Zvfhmin'.

This is based on:
<https://github.com/riscv/riscv-v-spec/blob/master/v-spec.adoc#zvfhmin-vector-extension-for-minimal-half-precision-floating-point>
<https://github.com/riscv/riscv-v-spec/blob/master/v-spec.adoc#zvfh-vector-extension-for-half-precision-floating-point>

Despite not having any new instructions, it will be necessary since those
extensions are already implemented in GCC.

Note that however, in this commit, following dependencies are implemented.

1.  'Zvfhmin' -> 'Zve32f'
2.  'Zvfh' -> 'Zvfhmin' (not 'Zvfh' -> 'Zve32f' as in the documentation)
3.  'Zvfh' -> 'Zfhmin'

This is because the instructions and configurations supported by the
'Zvfh' extension is a strict superset of the 'Zvfhmin' extension and
'Zvfh' -> 'Zve32f' dependency is indirectly derived from that fact.

bfd/ChangeLog:

	* elfxx-riscv.c (riscv_implicit_subsets): Add implications
	related to 'Zvfh' and 'Zvfhmin' extensions.
	(riscv_supported_std_z_ext) Add 'Zvfh' and 'Zvfhmin' to the list.