[MC][ELF] fix newly added test

Summary:
Reland of
- r344197 "[MC][ELF] compute entity size for explicit sections"
- r344206 "[MC][ELF] Fix section_mergeable_size.ll"
after being reverted in r344278 due to build breakages from not
specifying a target triple.

Move test from test/CodeGen/Generic/ to test/MC/ELF/.
Add explicit target triple so we don't try to run
this test on non ELF targets.

Reported: https://reviews.llvm.org/D53056#1261707

Reviewers: fhahn, rnk, espindola, NoQ

Reviewed By: fhahn, rnk

Subscribers: NoQ, MaskRay, rengolin, emaste, arichardson, llvm-commits, pirama, srhines

Differential Revision: https://reviews.llvm.org/D53146

llvm-svn: 344360