[POWER] Vectorize BF16 GEMV #5072

Job	Run time
build (ubuntu-latest, gfortran, cmake)	42m 51s
build (ubuntu-latest, gfortran, make)	16m 28s
build (ubuntu-latest, flang, cmake)	13m 12s
build (ubuntu-latest, flang, make)	7m 16s
build (macos-latest, gfortran, cmake)	14m 12s
build (macos-latest, gfortran, make)	5m 56s
msys2 (UCRT64, int32, Release)	17m 36s
msys2 (UCRT64, int64, Release)	17m 2s
msys2 (MINGW32, int32, Release)	19m 17s
msys2 (CLANG64, int32, Release)	17m 28s
msys2 (CLANG64, int64, Release)	16m 43s
msys2 (CLANG32, int32, Release)	12m 12s
msys2 (UCRT64, int32, None, mingw-w64-ucrt-x86_64, fc)	15m 9s
cross_build (mips64el, mips64el-linux-gnuabi64, DYNAMIC_ARCH=1 TARGET=GENERIC)	5m 46s
cross_build (riscv64, riscv64-linux-gnu, TARGET=RISCV64_GENERIC)	10m 10s
cross_build (mipsel, mipsel-linux-gnu, TARGET=MIPS1004K)	5m 30s
cross_build (alpha, alpha-linux-gnu, TARGET=EV4)	4m 50s
	4h 1m 38s

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