EmbeddedPkg: Only run in CI for GCC5

EmbeddedPkg: Only run in CI for GCC5

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=4046

This package currently does not build on non-GCC toolchains.

This change adds the package to edk2 CI so it can benefit from
ongoing CI and only tests the package against GCC.

Cc: Leif Lindholm <quic_llindhol@quicinc.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Abner Chang <abner.chang@amd.com>
Cc: Daniel Schaefer <git@danielschaefer.me>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>