RISC-V: Wider conflicts with the XTheadVector extension

RISC-V: Wider conflicts with the XTheadVector extension

T-Head/XuanTie's "XTheadVector" is based on a draft version of the now
standard vector extensions and it conflicts with not just the "V"
extension but all of its subsets.

This commit makes "XTheadVector" conflict with the "Zve32x" extension,
which is the smallest subset of the standard vector extensions.  Still,
a reference to "V" is preserved in the error message
as the documentation suggests:

> Therefore, the XTheadVector extension and the V extension are
> in conflict.

cf. <https://github.com/XUANTIE-RV/thead-extension-spec/blob/2.3.0/xtheadvector.adoc>

Note that, instructions in the "XTHeadZvamo" extension currently has
no conflicts with other extensions, even after addition of the "Zabha"
extension.

bfd/ChangeLog:

	* elfxx-riscv.c (riscv_parse_check_conflicts):
	Make "XTheadVector" conflict with not just "V" but its subsets.

gas/ChangeLog:

	* testsuite/gas/riscv/x-thead-vector-fail.d: Test a vector
	subset "Zve32x" to test failures.
	* testsuite/gas/riscv/x-thead-vector-fail.l: Reflect change
	in the error message.