Add OpenACC 2.6 `acc_get_property' support: restore Intel MIC offloading

Add OpenACC 2.6 `acc_get_property' support: restore Intel MIC offloading

The "OpenACC 2.6 `acc_get_property' support" changes regressed the relevant
libgomp OpenMP execution test cases to no longer consider Intel MIC offloading
because of:

    libgomp: while loading libgomp-plugin-intelmic.so.1: [...]/libgomp-plugin-intelmic.so.1: undefined symbol: GOMP_OFFLOAD_get_property

	liboffloadmic/
	* plugin/libgomp-plugin-intelmic.cpp (GOMP_OFFLOAD_get_property):
	New function.

(cherry picked from openacc-gcc-9-branch commit
1fa609ba73e9990ae7a65b083047f0ee219167b3)