Add and use vectorized packing in ZVL128B and ZVL256B for RISCV #5992

Job	Run time
cross_build (riscv64, riscv64-linux-gnu, TARGET=RISCV64_GENERIC)	3m 25s
cross_build (mipsel, mipsel-linux-gnu, TARGET=MIPS1004K)	4m 6s
cross_build (mips64el, mips64el-linux-gnuabi64, DYNAMIC_ARCH=1 TARGET=GENERIC)	4m 31s
neoverse_build	7m 37s
cross_build (alpha, alpha-linux-gnu, TARGET=EV4)	3m 27s
build (ubuntu-latest, flang, make)	9m 18s
build (ubuntu-latest, gfortran, cmake)	44m 58s
build (ubuntu-latest, gfortran, make)	21m 40s
build (ubuntu-latest, flang, cmake)	18m 25s
msys2 (UCRT64, int64, Release)	45m 49s
build (macos-latest, gfortran, make)	12m 46s
build (macos-latest, gfortran, cmake)	20m 19s
msys2 (CLANG64, int32, Release)	31m 31s
msys2 (CLANG64, int64, Release)	30m 44s
msys2 (MINGW32, int32, Release)	38m 2s
msys2 (UCRT64, int32, None, mingw-w64-ucrt-x86_64, fc)	41m 23s
msys2 (UCRT64, int32, Release)	45m 55s
	6h 23m 56s

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