#18378 Add s390 support for gold - v2

[Marcin Kościelnicki]

The copyright assignment process is now finished. I've done some
testing during last 2 weeks and made some fixes to the code.
Here comes the improved patchset.

Patch 1 adds support for 64-bit SHT_HASH entries, necessary for
s390. Patch 2 adds support for 64-bit archive symbol index, which
is used on s390 targets. Patch 3 adds the s390 backend proper.
Patch 4 fixes alignment in mergeable string sections, which causes
crashes on s390. Patches 5 and 6 fix the testsuite to pass on s390.