[ELF] Fix displacement computation for intra-section branch after D127611

D127611 computed st_value is inaccurate:

* For a backward branch, the destination address may be wrong if there is no
  relaxable relocation between it and the current location due to `if (remove)`.
  We may incorrectly relax a branch to c.j which ends up an overflow.
* For a forward branch, the destination address may be overestimated
  and lose relaxation opportunities.

To fix the issues,

* Don't reset st_value to the original value.
* Save the st_value delta from the previous iteration into valueDelta, and use
  `sa[0].d->value -= delta - valueDelta.find(sa[0].d)->second`.