[efi] Disable static assertions in EFI headers on non-EFI platforms

[efi] Disable static assertions in EFI headers on non-EFI platforms

The EDK2 headers may be included even in builds for non-EFI platforms.
Commits such as 9de6c45 ("[arm] Use -fno-short-enums for all 32-bit
ARM builds") have so far ensured that the compile-time checks within
the EDK2 headers will pass even when building for a non-EFI platform.

As a more general solution, temporarily disable static assertions
while including UefiBaseType.h if building on a non-EFI platform.
This avoids the need to modify the ABI on other platforms.

Signed-off-by: Michael Brown <mcb30@ipxe.org>