RISC-V: Add actual 'Zvkt' extension support

The 'Zvkt' extension is listed on the added extensions in the GNU Binutils
version 2.41 (see binutils/NEWS).  However, the support of this extension
was actually missing.

This commit adds actual support of this extension and adds implications
from 'Zvkn' and 'Zvks' superset extensions.

bfd/ChangeLog:

	* elfxx-riscv.c (riscv_implicit_subsets) Add implications from
	'Zvkn' and 'Zvks'.  (riscv_supported_std_z_ext): Add 'Zvkt' to
	the supported extension list.