<bit>: Test _Countl_zero_bsr() internal helper

[StephanTLavavej]

Recommended by Charlie Barto (@barcharcraz) in #1108 (comment) .

After merging #1108, <bit> is going to have an internal helper _Countl_zero_bsr() for x86/x64 that's active only for very old processors; therefore it will receive no regular testing, although it is necessary for correctness. While we've manually tested it for this PR, we should investigate adding automated testing by directly calling the helper (which we generally avoid doing, but this merits a special exception).

[AlexGuteniev]

I think a better alternative would be to have a fixture via force include, similar to this one:
https://github.com/microsoft/STL/blob/4e97255b3595a01f5268b98fad397e8579d14ed2/tests/std/include/force_include.hpp

It would add __isa_available = 0 (and __isa_enabled=0 on x86) in constructor of a static variable.

It would participate in test matrix.

A way to build such matrix for <bit> tests should be found, it will also be useful in the context of #954 resolution