[cmake] Make libgomp & libiomp5 alias install optional

Introduce a new LIBOMP_INSTALL_VARIABLES cache variable that can be used
to disable creating libgomp and libiomp5 aliases on 'make install'.
Those aliases are undesired e.g. on Gentoo systems where libomp is used
purely by clang.

Differential Revision: https://reviews.llvm.org/D24563

llvm-svn: 281512