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.

bfd/elfxx-riscv.c

@@ -1110,6 +1110,9 @@ static struct riscv_implicit_subset riscv_implicit_subsets[] =
   {"v", "d",		check_implicit_always},
   {"v", "zve64d",	check_implicit_always},
   {"v", "zvl128b",	check_implicit_always},
+  {"zvfh", "zvfhmin",	check_implicit_always},
+  {"zvfh", "zfhmin",	check_implicit_always},
+  {"zvfhmin", "zve32f",	check_implicit_always},
   {"zve64d", "d",	check_implicit_always},
   {"zve64d", "zve64f",	check_implicit_always},
   {"zve64f", "zve32f",	check_implicit_always},
@@ -1287,6 +1290,8 @@ static struct riscv_supported_ext riscv_supported_std_z_ext[] =
   {"zve64d",		ISA_SPEC_CLASS_DRAFT,		1, 0,  0 },
   {"zvbb",		ISA_SPEC_CLASS_DRAFT,		1, 0,  0 },
   {"zvbc",		ISA_SPEC_CLASS_DRAFT,		1, 0,  0 },
+  {"zvfh",		ISA_SPEC_CLASS_DRAFT,		1, 0,  0 },
+  {"zvfhmin",		ISA_SPEC_CLASS_DRAFT,		1, 0,  0 },
   {"zvkg",		ISA_SPEC_CLASS_DRAFT,		1, 0,  0 },
   {"zvkn",		ISA_SPEC_CLASS_DRAFT,		1, 0,  0 },
   {"zvkng",		ISA_SPEC_CLASS_DRAFT,		1, 0,  0 },