x86: Update memcpy/memset inline strategies for Skylake family CPUs

x86: Update memcpy/memset inline strategies for Skylake family CPUs

Simply memcpy and memset inline strategies to avoid branches for
Skylake family CPUs:

1. With MOVE_RATIO and CLEAR_RATIO == 17, GCC will use integer/vector
   load and store for up to 16 * 16 (256) bytes when the data size is
   fixed and known.
2. Inline only if data size is known to be <= 256.
   a. Use "rep movsb/stosb" with simple code sequence if the data size
      is a constant.
   b. Use loop if data size is not a constant.
3. Use memcpy/memset libray function if data size is unknown or > 256.

On Cascadelake processor with -march=native -Ofast -flto,

1. Performance impacts of SPEC CPU 2017 rate are:

500.perlbench_r  0.17%
502.gcc_r       -0.36%
505.mcf_r        0.00%
520.omnetpp_r    0.08%
523.xalancbmk_r -0.62%
525.x264_r       1.04%
531.deepsjeng_r  0.11%
541.leela_r     -1.09%
548.exchange2_r -0.25%
557.xz_r         0.17%
Geomean         -0.08%

503.bwaves_r     0.00%
507.cactuBSSN_r  0.69%
508.namd_r      -0.07%
510.parest_r     1.12%
511.povray_r     1.82%
519.lbm_r        0.00%
521.wrf_r       -1.32%
526.blender_r   -0.47%
527.cam4_r       0.23%
538.imagick_r   -1.72%
544.nab_r       -0.56%
549.fotonik3d_r  0.12%
554.roms_r       0.43%
Geomean          0.02%

2. Significant impacts on eembc benchmarks are:

eembc/idctrn01   9.23%
eembc/nnet_test  29.26%

gcc/

	* config/i386/x86-tune-costs.h (skylake_memcpy): Updated.
	(skylake_memset): Likewise.
	(skylake_cost): Change CLEAR_RATIO to 17.
	* config/i386/x86-tune.def (X86_TUNE_PREFER_KNOWN_REP_MOVSB_STOSB):
	Replace m_CANNONLAKE, m_ICELAKE_CLIENT, m_ICELAKE_SERVER,
	m_TIGERLAKE and m_SAPPHIRERAPIDS with m_SKYLAKE and m_CORE_AVX512.

gcc/testsuite/

	* gcc.target/i386/memcpy-strategy-9.c: New test.
	* gcc.target/i386/memcpy-strategy-10.c: Likewise.
	* gcc.target/i386/memcpy-strategy-11.c: Likewise.
	* gcc.target/i386/memset-strategy-7.c: Likewise.
	* gcc.target/i386/memset-strategy-8.c: Likewise.
	* gcc.target/i386/memset-strategy-9.c: Likewise.