ArmPkg/ArmSoftfloatLib: restrict -fno-tree-vrp option to GCC46 and GCC47

ArmPkg/ArmSoftfloatLib: restrict -fno-tree-vrp option to GCC46 and GCC47

The -fno-tree-vrp option is not required for GCC 4.8 or later, and is not
supported by CLANG. So restrict its use to GCC 4.6 and 4.7, which are the
oldest versions we support for ARM.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19283 6f19259b-4bc3-4df7-8a09-765794883524