Infer appropriate GNU_STACK alignment for a shared library.
Remove GNU_STACK segment alignment constants from elf traits, and
instead infer them from the linker (gold, or ld) used to build the
shared library whose relocations are being packed or unpacked.
In practice Android ignores any GNU_STACK segment. However, preserving
the values across a pack-and-unpack round trip helps to ensure that
the round trip library is bit-identical to the original input file.
BUG=385553
Review URL: https://codereview.chromium.org/
912403002
Cr-Commit-Position: refs/heads/master@{#315801}