Work around gcc15.1 on POWER misoptimizing DGEMV at -O3 #5968

Job	Run time
cross_build (alpha, alpha-linux-gnu, TARGET=EV4)	4m 6s
cross_build (mips64el, mips64el-linux-gnuabi64, DYNAMIC_ARCH=1 TARGET=GENERIC)	3m 40s
cross_build (riscv64, riscv64-linux-gnu, TARGET=RISCV64_GENERIC)	2m 48s
msys2 (CLANG64, int64, Release)	15m 24s
msys2 (MINGW32, int32, Release)	15m 29s
msys2 (UCRT64, int64, Release)	16m 44s
neoverse_build	7m 33s
msys2 (CLANG64, int32, Release)	20m 27s
build (ubuntu-latest, gfortran, make)	25m 46s
cross_build (mipsel, mipsel-linux-gnu, TARGET=MIPS1004K)	3m 18s
msys2 (UCRT64, int32, Release)	15m 10s
msys2 (UCRT64, int32, None, mingw-w64-ucrt-x86_64, fc)	12m 15s
build (macos-latest, gfortran, cmake)	17m 43s
build (ubuntu-latest, gfortran, cmake)	45m 21s
build (ubuntu-latest, flang, make)	17m 42s
build (macos-latest, gfortran, make)	4m 29s
build (ubuntu-latest, flang, cmake)	46m 26s
	4h 34m 21s

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