Since commit 20ab5045 (FindBLAS: Do not statically link against iomp5
in the case of Intel MKL, 2021-04-11), we no longer find MKL's BLAS when
using the GNU compiler because FindOpenMP chooses libgomp instead of
libiomp5, and mkl_intel_thread depends on the latter.  Revert the change
for now.  A new approach will be needed to solve the original problem.

Issue: #21811