[LLD] Remove unwanted --check-prefix=CHECK from unit tests. NFC.

Summary:
Removed unwanted --check-prefix=CHECK from the following unit tests:
      ELF/aarch64-gnu-ifunc.s
      ELF/gnu-ifunc-i386.s
      ELF/gnu-ifunc.s
      ELF/plt-i686.s

Patch by: Mandeep Singh Grang (mgrang)

Reviewers: rafael

Subscribers: aemerson

Projects: #lld

Differential Revision: http://reviews.llvm.org/D19149

llvm-svn: 266453