Setting `GEMM_PREFERED_SIZE` parameter for `A64FX` #5844

Job	Run time
cross_build (alpha, alpha-linux-gnu, TARGET=EV4)	3m 20s
cross_build (riscv64, riscv64-linux-gnu, TARGET=RISCV64_GENERIC)	3m 17s
cross_build (mips64el, mips64el-linux-gnuabi64, DYNAMIC_ARCH=1 TARGET=GENERIC)	4m 30s
cross_build (mipsel, mipsel-linux-gnu, TARGET=MIPS1004K)	4m 25s
msys2 (UCRT64, int32, Release)	45m 2s
msys2 (MINGW32, int32, Release)	37m 38s
msys2 (CLANG64, int32, Release)	30m 29s
msys2 (UCRT64, int64, Release)	44m 6s
build (ubuntu-latest, gfortran, cmake)	46m 3s
build (macos-latest, gfortran, cmake)	18m 12s
msys2 (CLANG64, int64, Release)	31m 24s
msys2 (UCRT64, int32, None, mingw-w64-ucrt-x86_64, fc)	40m 9s
build (ubuntu-latest, flang, make)	8m 2s
build (ubuntu-latest, flang, cmake)	17m 43s
neoverse_build	8m 10s
build (ubuntu-latest, gfortran, make)	21m 15s
build (macos-latest, gfortran, make)	11m 31s
	6h 15m 16s

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