cpu-supports: a module to honor GLIBC_TUNABLES=glibc.cpu.hwcaps

cpu-supports: a module to honor GLIBC_TUNABLES=glibc.cpu.hwcaps

This functionality is useful to allow better test coverage at least,
and may be useful for users to tune their environment,
avoiding CPU throttling for example.

* lib/cpu-supports.h (cpu_supports): A new wrapper that
checks that the GLIBC_TUNABLES environment variable allows
the hardware feature, before checking with __builtin_cpu_supports().
(cpu_may_support): Only perform the GLIBC_TUNABLES check,
which is useful if using other interfaces like getauxval().
(gcc_feature_to_glibc_hwcap): An internal helper that will resolve
at compile time with standard optimizations enabled.
* lib/cpu-supports.c (hwcap_allowed): Query the GLIBC_TUNABLES
environment variable (read once per process), to see if the
passed GLIBC_HWCAP is allowed.
* modules/cpu-supports: New module definition.
* modules/cpu-supports-tests: New test module definition.
* tests/test-cpu-supports.c: New tests.