StdLib: remove mention of ARMGCC

StdLib: remove mention of ARMGCC

The ARMGCC toolchain will be removed, and so will the build rule family
by the same name. So remove the BuildOptions specific to ARMGCC.

Cc: Daryl McDaniel <edk2-lists@mc2research.org>
Cc: Jaben Carsey <jaben.carsey@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Daryl McDaniel <edk2-lists@mc2research.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18207 6f19259b-4bc3-4df7-8a09-765794883524