xlat: check that system definitions match fallback definitions

xlat: check that system definitions match fallback definitions

For each constant that is accompanied by a fallback definition,
generate a sanity check that the definition provided by system headers
matches the fallback definition.

* gcc_compat.h (DIAG_PUSH_IGNORE_TAUTOLOGICAL_COMPARE,
DIAG_POP_IGNORE_TAUTOLOGICAL_COMPARE): New macros.
* xlat/gen.sh (cond_def): Generate a code that checks the system
definition against the default.
(gen_header): Generate a code that includes "gcc_compat.h" and
"static_assert.h".

Co-Authored-by: Eugene Syromyatnikov <evgsyr@gmail.com>