[ELF] - Extend edata-etext.s test case.

We did not have a test that would test that
_etext address is equal to etext, _end == end and _edata == edata.

Because of that, the following line was never
executed when running our tests:
https://github.com/llvm-mirror/lld/blob/master/ELF/Writer.cpp#L928

Patch fixes that.

llvm-svn: 336280