gcc: Enable OpenMP compiling and library

gcc: Enable OpenMP compiling and library

Multicore embedded systems are getting more and more common.

Remove "--disable-openmp" from the GCC configuration options and
always build libgomp. This only creates a "bigger" compiler but
has no effect on the compiled binaries that don't use openmp.

Tested a clean build on mips32el and arm7a, no problems encountered.

Autoconf will not detect OpenMP after this change, because it will
build and run a target binary on the build system. In order to use
OpenMP, the variable ac_cv_prog_c_openmp=-fopenmp must be set.

(From OE-Core rev: fdf37b792c13a1fc3f63faf471f0403396414a35)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>