[lld][ELF] Removing redundant cast. NFC.

Also a couple of minor cleanups in merge-string.s:
- fix inconsistent use of tabs
- use `.p2align` rather than `.align` since `.p2align` works the
  same on all platforms (the meaning of align seems to differ
  between platforms according to `AlignmentIsInBytes`.

I noticed these potential cleanups while porting SHF_STRINGS support to
wasm-ld.

Differential Revision: https://reviews.llvm.org/D97647