Disable GEMM3M for generic targets (not implemented) #4678

Job	Run time
build (ubuntu-latest, gfortran, cmake)	44m 6s
build (ubuntu-latest, gfortran, make)	16m 35s
cross_build (mips64el, mips64el-linux-gnuabi64, DYNAMIC_ARCH=1 TARGET=GENERIC)	3m 47s
build (ubuntu-latest, flang, cmake)	11m 36s
cross_build (riscv64, riscv64-linux-gnu, TARGET=RISCV64_GENERIC)	8m 22s
build (ubuntu-latest, flang, make)	6m 30s
cross_build (mipsel, mipsel-linux-gnu, TARGET=MIPS1004K)	3m 27s
msys2 (UCRT64, int32, Release)	17m 52s
build (macos-latest, gfortran, cmake)	1h 26m 45s
cross_build (alpha, alpha-linux-gnu, TARGET=EV4)	2m 58s
msys2 (UCRT64, int64, Release)	19m 7s
build (macos-latest, gfortran, make)	3m 34s
msys2 (MINGW32, int32, Release)	25m 30s
msys2 (CLANG64, int32, Release)	18m 41s
msys2 (CLANG64, int64, Release)	16m 26s
msys2 (CLANG32, int32, Release)	12m 32s
msys2 (UCRT64, int32, None, mingw-w64-ucrt-x86_64, fc)	15m 30s
	5h 13m 18s

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