Software mitigation: Disable gather generation in vectorization for GDS affected Intel Processors.

For more details of GDS (Gather Data Sampling), refer to
https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/advisory-guidance/gather-data-sampling.html

After microcode update, there's performance regression. To avoid that,
the patch disables gather generation in autovectorization but uses
gather scalar emulation instead.

gcc/ChangeLog:

	* config/i386/i386-options.cc (m_GDS): New macro.
	* config/i386/x86-tune.def (X86_TUNE_USE_GATHER_2PARTS): Don't
	enable for m_GDS.
	(X86_TUNE_USE_GATHER_4PARTS): Ditto.
	(X86_TUNE_USE_GATHER): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/avx2-gather-2.c: Adjust options to keep
	gather vectorization.
	* gcc.target/i386/avx2-gather-6.c: Ditto.
	* gcc.target/i386/avx512f-pr88464-1.c: Ditto.
	* gcc.target/i386/avx512f-pr88464-5.c: Ditto.
	* gcc.target/i386/avx512vl-pr88464-1.c: Ditto.
	* gcc.target/i386/avx512vl-pr88464-11.c: Ditto.
	* gcc.target/i386/avx512vl-pr88464-3.c: Ditto.
	* gcc.target/i386/avx512vl-pr88464-9.c: Ditto.
	* gcc.target/i386/pr88531-1b.c: Ditto.
	* gcc.target/i386/pr88531-1c.c: Ditto.