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.

gcc/testsuite/ChangeLog

+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.
+
 2020-03-25  Mark Eggleston  <mark.eggleston@codethink.com>
 
 	PR fortran/93484

gcc/testsuite/gfortran.dg/iso_c_binding_compiler_1.f90

 ! { dg-do link }
+! The compiler_options() function is dependent on the
+! command line options and thus incompatible with -fcompare-debug.
+! { dg-skip-if "-fcompare-debug incompatible test" { *-*-* } { "-fcompare-debug" } { "" } } */
 !
 ! PR fortran/40569
 !

gcc/testsuite/gfortran.dg/iso_c_binding_compiler_3.f90

 ! { dg-do compile }
 ! { dg-options "-Wall" }
+! The compiler_options() function is dependent on the
+! command line options and thus incompatible with -fcompare-debug.
+! { dg-skip-if "-fcompare-debug incompatible test" { *-*-* } { "-fcompare-debug" } { "" } } */
 !
 ! PR fortran/45823
 !

gcc/testsuite/gfortran.dg/unlimited_polymorphic_31.f03

 ! { dg-do run }
+! The compiler_options() function is dependent on the
+! command line options and thus incompatible with -fcompare-debug.
+! { dg-skip-if "-fcompare-debug incompatible test" { *-*-* } { "-fcompare-debug" } { "" } } */
 !
 ! Test the fix for PR92785, where the array passed to 'write scalar' was not
 ! normalised to LBOUND = 1.