CMake: Demote the OpenMP mixed linkage check to NOTICE #6030

Job	Run time
neoverse_build	7m 44s
cross_build (mips64el, mips64el-linux-gnuabi64, DYNAMIC_ARCH=1 TARGET=GENERIC)	3m 46s
cross_build (mipsel, mipsel-linux-gnu, TARGET=MIPS1004K)	3m 34s
build (ubuntu-latest, gfortran, make)	22m 53s
cross_build (alpha, alpha-linux-gnu, TARGET=EV4)	2m 43s
cross_build (riscv64, riscv64-linux-gnu, TARGET=RISCV64_GENERIC)	2m 51s
msys2 (UCRT64, int32, Release)	13m 44s
build (ubuntu-latest, flang, cmake)	10m 24s
build (ubuntu-latest, gfortran, cmake)	47m 0s
build (ubuntu-latest, flang, make)	6m 14s
msys2 (MINGW32, int32, Release)	15m 26s
build (macos-latest, gfortran, cmake)	28s
build (macos-latest, gfortran, make)	28s
msys2 (CLANG64, int64, Release)	18m 8s
msys2 (CLANG64, int32, Release)	15m 26s
msys2 (UCRT64, int64, Release)	14m 5s
msys2 (UCRT64, int32, None, mingw-w64-ucrt-x86_64, fc)	11m 44s
	3h 16m 38s

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