[ELF] - Inform user that we do not support dynamic relocations against text segment.

It was reported in PR28020, that lld does not link code which
gold do. But in fact that is expected behavior as we do not
support DT_TEXTREL.

This patch changes error message as it can report about relocations against
text segments exclusively, other dynamic relocations errors can 
be handled separately.

Differential revision: http://reviews.llvm.org/D21133

llvm-svn: 272377