Clang does not support undoing the effects of -mstrict-align by passing
the -mno-strict-align counterpart, so appending the latter to the
compiler's XIPFLAGS does not work. Instead, clear the flags entirely.
This also removes -mgeneral-regs-only, but this is fine - we can
tolerate SIMD codegen in PEIMs or BASE libraries as they run with the
MMU and caches enabled.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>