StandaloneMmPkg: Support CLANGPDB builds

StandaloneMmPkg: Support CLANGPDB builds

Currently, the flag "-fpie" is passed for all builds with a GCC
family toolchain, including CLANGPDB. CLANGPDB however does not
support this flag as it generates PE/COFF files directly.

As the flag is mostly required for ARM-specific self-relocation, drop
it for other architectures and document the limitation to enable e.g.
X64 CLANGPDB builds of StandaloneMmCore.

Signed-off-by: Marvin Häuser <mhaeuser@posteo.de>
Acked-by: Shi Steven <steven.shi@intel.com>