Adjust makefiles for GCC80 update.

gcc80: Add local -save-temps=objects option.

The gcc currently supports -save-temps= cwd, obj, object(not documented).
None of those variants can be used at all during buildworld sequence,
even at s

gcc80: Add local -save-temps=objects option.

The gcc currently supports -save-temps= cwd, obj, object(not documented).
None of those variants can be used at all during buildworld sequence,
even at serial buildworld, without interfering with build in funny ways.
This is very easy to notice while compiling libraries, where rules are:
* foo.c --> foo.o (non-pic)
* foo.c --> foo.So (pic)
* foo.c --> foo.po (profiled)

The added option forces to use full object name (if it was supplied)
when constructing names for .s and .i intermediates, as follows:
cc -c foo.c -o foo.o --> foo.o.s + foo.o.i
This allows to use -save-temps globally during buildworld and it is
*less* likely to interfere while producing normally created objects.
Repeated quickworld likely *will* have side effects, because the '-o'
flags are not used for kernel and programs (foo.s will shadow foo.c).

Tested by binary comparing /usr/obj trees for differences.

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gcc80: Document changes in contrib sources.

Add READMEs for new vendor/GCC80 branch.

Next planned major gcc version update is gcc-11.0 in next three years.
Roadmap: gcc34 -> gcc41 -> gcc44 -> gcc47 -> gcc50 -> gcc80 [-> gcc11]