Silicon/, Platform/: Get rid of redundant toolchains flags

The ARCHCC and ARCHASM flags are redundant, given that ARMv7-A with
Thumb2 codegen has been the default for a long time, for all supported
toolchains.

So let's drop these definitions: this allows us to retire ARCHCC and
ARCHASM entirely from EDK2.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>