bug#28213: [bootstrap] Error 2 with -Os with gcc (GCC) 7.1.1

[Paul Eggert]

Thanks for the bug report, which led to an entertaining and hair-raising 
trip through Emacs's garbage collector and let me brush up on my 
machine-level debugging skills. I installed the attached patches, which 
fix the problem for me, and I am closing the bug.

This bug report underscores that --enable-link-time-optimization is 
experimental and should not yet be recommended for production use with 
Emacs. When used, even with the attached patches I can easily get Emacs 
to dump core by exploiting bugs in GCC's code generator. The commit 
message in the last of the attached patches contains more detail about this.

0001-Align-stack-bottom-properly.patch
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0002-Improve-stack-top-heuristic.patch
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0003-Make-garbage-collection-more-conservative.patch
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0004-Be-more-conservative-in-link-time-optimization-doc.patch
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