Improve [SD]SYEVD performance by parallelizing [SD]LAED3 #5855

Job	Run time
build (macos-latest, gfortran, make)	9m 37s
build (ubuntu-latest, gfortran, make)	22m 57s
neoverse_build	7m 34s
msys2 (UCRT64, int64, Release)	1h 41m 4s
build (macos-latest, gfortran, cmake)	27m 36s
build (ubuntu-latest, flang, cmake)	39m 8s
msys2 (UCRT64, int32, Release)	1h 40m 29s
msys2 (MINGW32, int32, Release)	1h 0m 37s
build (ubuntu-latest, flang, make)	16m 21s
build (ubuntu-latest, gfortran, cmake)	45m 8s
msys2 (UCRT64, int32, None, mingw-w64-ucrt-x86_64, fc)	1h 25m 35s
msys2 (CLANG64, int32, Release)	1h 6m 18s
cross_build (mipsel, mipsel-linux-gnu, TARGET=MIPS1004K)	6m 15s
msys2 (CLANG64, int64, Release)	1h 7m 32s
cross_build (riscv64, riscv64-linux-gnu, TARGET=RISCV64_GENERIC)	5m 11s
cross_build (mips64el, mips64el-linux-gnuabi64, DYNAMIC_ARCH=1 TARGET=GENERIC)	7m 7s
cross_build (alpha, alpha-linux-gnu, TARGET=EV4)	5m 39s
	11h 14m 8s

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