[PATCH, libgfortran] proposed fix for SPEC CPU2017 621.wrf_s failures

[Jim Wilson]

As mentioned in bug 81195, I see openmp related failures due to a lack
of locking of the newunit_stack and newunit_tos variables.  The code
locks when pushing onto the stack, but does not lock when popping from
the stack.  This can cause multiple threads to pop the same structure,
which then eventually leads to an error.

This patch was tested with an aarch64 bootstrap and make check.  There
were no regressions.  It was also tested with a SPEC CPU2017 run, and
solves the 621.wrf_s failure I get without the patch.

gcc 7 has the same problem.  gcc 6 is OK.

Jim
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