[ELF] -pie: produce dynamic relocations for absolute relocations referencing undef weak

See the comment for my understanding of -no-pie and -shared expectation.
-no-pie has freedom on choices. We choose dynamic relocations to be consistent
with the handling of GOT-generating relocations.

Note: GNU ld has arch-varying behaviors and its x86 -pie has a very
complex rule:
if there is at least one GOT-generating or PLT-generating relocation and
-z dynamic-undefined-weak (enabled by default) is in effect, generate a
dynamic relocation.

We don't emulate its rule.

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D105164