external-toolchain-cross: don't wrap binaries that don't exist

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
*-external-cross: append -${TARGET_ARCH} to PN to match internal

This should quiet manifest not found warnings from autotools_copy_aclocals.

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
gcc-runtime-external: drop packaging of omp.h for now

Currently external-toolchain.bbclass doesn't support extraction of files with
this sort of pattern.

Signed-off-by: Christopher Larson <chris_larson@mentor.com>