Re-apply: Drop excess size used for run time allocated stack variables.

Re-apply: Drop excess size used for run time allocated stack variables.

The patch got reverted after hitting PR77359 which turned out to be a
rs6000 backend problem.  Reapplying after the PR got fixed.

gcc/ChangeLog:

2016-11-18  Dominik Vogt  <vogt@linux.vnet.ibm.com>

	Re-apply after PR bootstrap/77359 is fixed:
	2016-08-23  Dominik Vogt  <vogt@linux.vnet.ibm.com>

        * explow.c (get_dynamic_stack_size): Take known alignment of stack
        pointer + STACK_DYNAMIC_OFFSET into account when calculating the
        size needed.

--This line, and those below, will be
ignored--

M    gcc/ChangeLog
M    gcc/explow.c

From-SVN: r242590