[WIP][DO NOT Merge] SBEGEMM support for SVE256 #5115

Job	Run time
build (ubuntu-latest, gfortran, cmake)	41m 34s
build (ubuntu-latest, gfortran, make)	16m 42s
build (ubuntu-latest, flang, cmake)	44m 6s
build (ubuntu-latest, flang, make)	18m 44s
cross_build (mips64el, mips64el-linux-gnuabi64, DYNAMIC_ARCH=1 TARGET=GENERIC)	6m 38s
msys2 (UCRT64, int32, Release)	54m 12s
build (macos-latest, gfortran, cmake)	32m 14s
cross_build (riscv64, riscv64-linux-gnu, TARGET=RISCV64_GENERIC)	10m 41s
msys2 (UCRT64, int64, Release)	50m 53s
build (macos-latest, gfortran, make)	6m 28s
cross_build (mipsel, mipsel-linux-gnu, TARGET=MIPS1004K)	6m 12s
msys2 (MINGW32, int32, Release)	41m 29s
cross_build (alpha, alpha-linux-gnu, TARGET=EV4)	5m 29s
msys2 (CLANG64, int32, Release)	29m 48s
msys2 (CLANG64, int64, Release)	31m 48s
msys2 (CLANG32, int32, Release)	24m 2s
msys2 (UCRT64, int32, None, mingw-w64-ucrt-x86_64, fc)	51m 1s
	7h 52m 1s

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