[LLD][ELF] - Fix the different behavior of the linker script symbols on different platforms.

This generalizes code and also fixes the broken behavior shown in
one of our test cases for some targets, like x86-64.

The issue occurs when the forward declarations are used in the script.
One of the samples is:

SECTIONS {
  foo = ADDR(.text) - ABSOLUTE(ADDR(.text));
};

In that case, we have a broken output when output target does
not use thunks. That happens because thunks creating code
(called from maybeAddThunks)
calls Script->assignAddresses() at least one more time,
what fixups the values. As a result final symbols values can
be different on AArch64 and x86, for example.

In this patch, I generalize and rename maybeAddThunks to
finalizeAddressDependentContent and now it is used and called
by all targets.

Differential revision: https://reviews.llvm.org/D55550

llvm-svn: 358646