Use fld.d/fst.d in PROLOGUE/EPILOGUE in LOONGSON3R5 GEMM #4979

Job	Run time
build (ubuntu-latest, gfortran, cmake)	41m 40s
build (ubuntu-latest, gfortran, make)	16m 46s
build (ubuntu-latest, flang, cmake)	44m 24s
build (ubuntu-latest, flang, make)	19m 8s
build (macos-latest, gfortran, cmake)	15m 29s
build (macos-latest, gfortran, make)	6m 32s
cross_build (mips64el, mips64el-linux-gnuabi64, DYNAMIC_ARCH=1 TARGET=GENERIC)	5m 51s
cross_build (riscv64, riscv64-linux-gnu, TARGET=RISCV64_GENERIC)	9m 57s
cross_build (mipsel, mipsel-linux-gnu, TARGET=MIPS1004K)	5m 44s
cross_build (alpha, alpha-linux-gnu, TARGET=EV4)	4m 48s
msys2 (UCRT64, int32, Release)	15m 32s
msys2 (UCRT64, int64, Release)	16m 35s
msys2 (MINGW32, int32, Release)	19m 35s
msys2 (CLANG64, int32, Release)	18m 24s
msys2 (CLANG64, int64, Release)	16m 2s
msys2 (CLANG32, int32, Release)	9m 33s
msys2 (UCRT64, int32, None, mingw-w64-ucrt-x86_64, fc)	17m 29s
	4h 43m 29s

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