Find .plt section in object files generated by recent ld

Code in ObjectFileELF::ParseTrampolineSymbols assumes that the sh_info
field of the .rel(a).plt section identifies the .plt section.

However, with recent GNU ld this is no longer true.  As a result of this:
https://sourceware.org/bugzilla/show_bug.cgi?id=18169
in object files generated with current linkers the sh_info field of
.rel(a).plt now points to the .got.plt section (or .got on some targets).

This causes LLDB to fail to identify any PLT stubs, causing a number of
test case failures.

This patch changes LLDB to simply always look for the .plt section by
name.  This should be safe across all linkers and targets.

Differential Revision: http://reviews.llvm.org/D18973

llvm-svn: 266316