libgfortran: Fix build on non-glibc targets

libgfortran: Fix build on non-glibc targets

When the __GLIBC_PREREQ macro isn't defined, the
  #if ... && defined __GLIBC_PREREQ && __GLIBC_PREREQ (2, 32)
directive has invalid syntax - the __GLIBC_PREREQ in there evaluates
to 0 and is followed by (2, 32).

2022-01-12  Jakub Jelinek  <jakub@redhat.com>

	* libgfortran.h (POWER_IEEE128): Use __GLIBC_PREREQ in a separate
	#if directive inside of #if ... && defined __GLIBC_PREREQ.