IBM Z: Fix bootstrap breakage due to HAVE_TF macro

IBM Z: Fix bootstrap breakage due to HAVE_TF macro

Commit e627cda56865 ("IBM Z: Store long doubles in vector registers
when possible") introduced HAVE_TF macro which expands to a logical
"or" of HAVE_ constants.  Not all of these constants are available in
GENERATOR_FILE context, so a hack was used: simply expand to true in
this case, because the actual value matters only during compiler
runtime and not during generation.

However, one aspect of this value matters during generation after all:
whether or not it's a constant, which in this case it appears to be.
This results in incorrect values in insn-flags.h and broken bootstrap
for some configurations.

Fix by using a dummy value that is not a constant.

gcc/ChangeLog:

2020-11-10  Ilya Leoshkevich  <iii@linux.ibm.com>

	* config/s390/s390.h (HAVE_TF): Use opaque value when
	GENERATOR_FILE is defined.