OpenMP: Fix testsuite failure on x86 with -m32

The testcase decare-variant-duplicates.c added in commit
96246bff failed on 32-bit x86
because on that target "i386" is defined as a preprocessor macro
and cannot be used as an identifier.  Fixed by rewriting that test
not to do that.

gcc/testsuite/ChangeLog
	* c-c++-common/gomp/declare-variant-duplicates.c: Avoid using
	"i386" as an identifier.