ArmPkg/ArmV7Lib: add CLANG alternative for FPEXC access

ArmPkg/ArmV7Lib: add CLANG alternative for FPEXC access

The open coded access to co-processor #10 to set FPEXC is not supported
by the CLANG assembler, but the architecturally correct VMSR instruction
is not supported by older binutils. So keep the former unless __clang__
is defined.

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@19282 6f19259b-4bc3-4df7-8a09-765794883524