Disable gather/scatter for zen4

Disable gather/scatter for zen4

this patch adds more tunes for zen4:
 - new tunes for avx512 scater instructions.
   In micro benchmarks these seems consistent loss compared to open-coded coe
 - disable use of gather for zen4
   While these are win for a micro benchmarks (based on TSVC), enabling gather
   is a loss for parest. So for now it seems safe to keep it off.
 - disable pass to avoid FMA chains for znver4 since fmadd was optimized and does not seem
   to cause regressions.

	* config/i386/i386.cc (ix86_vectorize_builtin_scatter): Guard scatter
	by TARGET_USE_SCATTER.
	* config/i386/i386.h (TARGET_USE_SCATTER_2PARTS,
	TARGET_USE_SCATTER_4PARTS, TARGET_USE_SCATTER): New macros.
	* config/i386/x86-tune.def (TARGET_USE_SCATTER_2PARTS,
	TARGET_USE_SCATTER_4PARTS, TARGET_USE_SCATTER): New tunes.
	(X86_TUNE_AVOID_256FMA_CHAINS, X86_TUNE_AVOID_512FMA_CHAINS): Disable
	for znver4.  (X86_TUNE_USE_GATHER): Disable for zen4.