[ELF] Don't emit dynamic relocations with weak undef in writable sections

In processRelocAux(), our handling of 1) link-time constant and 2) weak
undef is the same, so put them together to simplify the logic.

This moves the weak undef code around. The result is that: in a writable
section (or -z notext), we will no longer emit dynamic relocations for
weak undefined symbols.

The new behavior seems to match GNU linkers, and improves consistency
with the case of a readonly section.

The condition `!Config->Shared` was there probably because it is common
for a -shared link not to specify full dependencies. Keep it now but we
may revisit the decision in the future.

gABI says:

> The behavior of weak symbols in areas not specified by this document is
> implementation defined. Weak symbols are intended primarily for use in
> system software. Applications using weak symbols are unreliable since
> changes in the runtime environment might cause the execution to fail.

Reviewed By: ruiu

Differential Revision: https://reviews.llvm.org/D63003

llvm-svn: 363399