testsuite: Fix up FAILs in gfortran testsuite with -fcompare-debug [PR94280]

testsuite: Fix up FAILs in gfortran testsuite with -fcompare-debug [PR94280]

These 3 tests use compiler_options() function,
which is inherently incompatible with -fcompare-debug compilation, as it
emits into a string literal in the assembly the exact f951 command line
options, which differs between the two compilations with -fcompare-debug,
where one has -gtoggle and -fcompare-debug-second options added and
different -fdump-final-insns= option argument.

The following patch adds dg-skip-if directives, so that these tests are
ignored during
make check-gfortran RUNTESTFLAGS='--target_board=unix/-fcompare-debug'

2020-03-25  Jakub Jelinek  <jakub@redhat.com>

	PR debug/94280
	* gfortran.dg/iso_c_binding_compiler_1.f90: Add dg-skip-if for
	-fcompare-debug.
	* gfortran.dg/iso_c_binding_compiler_3.f90: Likewise.
	* gfortran.dg/unlimited_polymorphic_31.f03: Likewise.