x86: support AVX10.1/512

Since this is merely a re-branding of certain AVX512* features, there's
little code to be added.

The main aspect here are new testcases. In order to be able to re-use
some of the existing testcases, several of them need their start symbols
adjusted. Note that 256- and 128-bit tests want adding here, as these
need to work right away. Subsequently they'll gain vector length
constraints.

Since it was missing and is wanted here, also add an AVX512VL+VPOPCNTDQ
test.