testsuite: Remove reliance on argc in lto/pr101868_0.c

testsuite: Remove reliance on argc in lto/pr101868_0.c

Some embedded targets do not pass any argv arguments.  When argc is
zero, this causes spurious failures for lto/pr101868_0.c.  Fix by
following the strategy in r0-114701-g2c49569ecea56d.  Use a volatile
variable instead of argc to inject a runtime value into the test.

I validated the following:
  - No changes in testresults for x86_64-pc-linux-gnu.
  - The spurious failures are fixed for PRU target.
  - lto/pr101868_0.c still fails on x86_64-pc-linux-gnu, if
    the PR/101868 fix (r12-2254-gfedcf3c476aff7) is reverted.

	PR tree-optimization/101868

gcc/testsuite/ChangeLog:

	* gcc.dg/lto/pr101868_0.c (zero): New volatile variable.
	(main): Use it instead of argc.

Signed-off-by: Dimitar Dimitrov <dimitar@dinux.eu>