StandaloneMmPkg: Fix build failure - Bug 2253

StandaloneMmPkg: Fix build failure - Bug 2253

The StandaloneMmPkg currently has code for supporting
Arm architecture only. Support for X64 and IA32 is
currently under development on a separate branch.

However, StandaloneMmPkg/StandaloneMmPkg.dsc is
indicating that support for X64 and IA32 is
available which is causing build failures.
This has been reported in
https://bugzilla.tianocore.org/show_bug.cgi?id=2253

This issue has been discussed earlier on the list:
(1) https://edk2.groups.io/g/devel/message/47276

(2) https://edk2.groups.io/g/devel/message/47283

In light of the above, this patch removes IA32 and X64
from SUPPORTED_ARCHITECTURES, until support for the
respective architectures is merged into StandaloneMmPkg.

Cc: Achin Gupta <achin.gupta@arm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Supreeth Venkatesh <supreeth.venkatesh@arm.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>