[ELF] Fall back to search dirs for linker scripts specified with -T

Summary:
This matches the behaviour of ld.bfd:
https://sourceware.org/binutils/docs/ld/Options.html#Options

If scriptfile does not exist in the current directory, ld looks for it in
the directories specified by any preceding '-L' options. Multiple '-T'
options accumulate.

Reviewers: ruiu, grimar

Reviewed By: ruiu, grimar

Subscribers: emaste, llvm-commits

Differential Revision: https://reviews.llvm.org/D40129

llvm-svn: 318655