IBM Z: Detect libc's float_t behavior on cross compiles

IBM Z: Detect libc's float_t behavior on cross compiles

When cross-compiling GCC with target libc headers available and
configure option --enable-s390-excess-float-precision has been omitted,
identify whether they clamp float_t to double or respect
__FLT_EVAL_METHOD__ via a compile test that coerces the build-system
compiler to use the target headers. Then derive the setting from that.

gcc/ChangeLog:

2020-12-16  Marius Hillenbrand  <mhillen@linux.ibm.com>

	* configure.ac: Change --enable-s390-excess-float-precision
	default behavior for cross compiles with target headers.
	* configure: Regenerate.
	* doc/install.texi: Adjust documentation.