[ELF] - Linkerscript: reimplement readSectionExcludes()

It is not only a bit more straightforward now, but also next 2 issues are solved:

* It just crashed on ".foo : { *(EXCLUDE_FILE (*file1.o)) }" before.
* It accepted multiple EXCLUDE_FILEs in a row.

Differential revision: https://reviews.llvm.org/D24726

llvm-svn: 282060