Fix building of bgemm tests on GEMM3M-capable (x86) targets #5964

Job	Run time
neoverse_build	8m 12s
cross_build (riscv64, riscv64-linux-gnu, TARGET=RISCV64_GENERIC)	7m 32s
build (ubuntu-latest, gfortran, make)	22m 1s
cross_build (alpha, alpha-linux-gnu, TARGET=EV4)	2m 53s
build (ubuntu-latest, gfortran, cmake)	47m 12s
build (ubuntu-latest, flang, cmake)	45m 12s
msys2 (UCRT64, int32, Release)	14m 53s
cross_build (mips64el, mips64el-linux-gnuabi64, DYNAMIC_ARCH=1 TARGET=GENERIC)	3m 59s
build (ubuntu-latest, flang, make)	18m 18s
build (macos-latest, gfortran, make)	12m 57s
build (macos-latest, gfortran, cmake)	16m 10s
msys2 (UCRT64, int64, Release)	1h 39m 55s
msys2 (CLANG64, int32, Release)	1h 10m 27s
msys2 (MINGW32, int32, Release)	15m 27s
cross_build (mipsel, mipsel-linux-gnu, TARGET=MIPS1004K)	3m 42s
msys2 (CLANG64, int64, Release)	1h 8m 30s
msys2 (UCRT64, int32, None, mingw-w64-ucrt-x86_64, fc)	1h 28m 35s
	9h 5m 55s

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