FindBLAS: Fix detection of OpenMP as dependency of BLA_STATIC

Update the change from commit f7f3d898 (FindBLAS: Add dependency of
OpenBLAS on OpenMP for BLA_STATIC, 2020-11-10, v3.20.0-rc1~492^2):

* If C is not enabled, find CXX OpenMP libraries instead.

* Do not use BLA_STATIC's custom CMAKE_FIND_LIBRARY_SUFFIXES for OpenMP.
  It can break projects that already call `find_package(OpenMP)` and
  expect a shared library.  Whether OpenMP is static is orthogonal to
  whether BLAS is static.

Fixes: #22039
Issue: #16221