IBM Z: Configure excess precision for float at compile-time

IBM Z: Configure excess precision for float at compile-time

Historically, float_t has been defined as double on s390 and gcc would
emit double precision insns for evaluating float expressions when in
standard-compliant mode. Configure that behavior at compile-time as prep
for changes in glibc: When glibc ties float_t to double, keep the old
behavior; when glibc derives float_t from FLT_EVAL_METHOD (as on most
other archs), revert to the default behavior (i.e.,
FLT_EVAL_METHOD_PROMOTE_TO_FLOAT). Provide a configure option
--enable-s390-excess-float-precision to override the check.

gcc/ChangeLog:

2020-12-01  Marius Hillenbrand  <mhillen@linux.ibm.com>

	* configure.ac: Add configure option
	--enable-s390-excess-float-precision and check to derive default
	from glibc.
	* config/s390/s390.c: Guard s390_excess_precision with an ifdef
	for ENABLE_S390_EXCESS_FLOAT_PRECISION.
	* doc/install.texi: Document --enable-s390-excess-float-precision.
	* configure: Regenerate.
	* config.in: Regenerate.