Add support for BF16 sbgemm on RISCV. #6084

Job	Run time
build (ubuntu-latest, gfortran, cmake)	47m 9s
cross_build (mips64el, mips64el-linux-gnuabi64, DYNAMIC_ARCH=1 TARGET=GENERIC)	9m 56s
build (ubuntu-latest, gfortran, make)	24m 0s
build (ubuntu-latest, flang, make)	20m 43s
build (macos-latest, gfortran, make)	13m 18s
build (ubuntu-latest, flang, cmake)	44m 24s
cross_build (alpha, alpha-linux-gnu, TARGET=EV4)	8m 17s
neoverse_build	8m 3s
cross_build (mipsel, mipsel-linux-gnu, TARGET=MIPS1004K)	8m 50s
msys2 (UCRT64, int32, Release)	1h 50m 3s
build (macos-latest, gfortran, cmake)	36m 7s
msys2 (CLANG64, int32, Release)	1h 18m 12s
msys2 (UCRT64, int64, Release)	1h 49m 30s
cross_build (riscv64, riscv64-linux-gnu, TARGET=RISCV64_GENERIC)	9m 19s
msys2 (UCRT64, int32, None, mingw-w64-ucrt-x86_64, fc)	1h 37m 11s
msys2 (MINGW32, int32, Release)	1h 15m 10s
msys2 (CLANG64, int64, Release)	1h 17m 56s
	12h 58m 8s

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