Add configure option to force enable 'soft' HW feature bits

* configure.ac (force_soft_hwfeatures)
(ENABLE_FORCE_SOFT_HWFEATURES): New.
* src/hwf-x86.c (detect_x86_gnuc): Enable HWF_INTEL_FAST_SHLD
and HWF_INTEL_FAST_VPGATHER if ENABLE_FORCE_SOFT_HWFEATURES enabled.

Patch allows enabling HW features, that are fast only select CPU models,
on all CPUs. For example, SHLD instruction is fast on only select Intel
processors and should not be used on others. This configuration option
allows enabling these 'soft' HW features for testing purposes on all
CPUs.

Current 'soft' HW features are:

• "intel-fast-shld": supported by all x86 (but very slow on most)
• "intel-fast-vpgather": supported by all x86 with AVX2 (but slow on most)

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>