Add .extab and .exidx sections

These sections are required by clang when the code is compiled for
aarch32. These sections are related to the unwind of the stack in
exceptions, but in the way that clang defines and uses them, the
garbage collector cannot get rid of them.

Change-Id: I085efc0cf77eae961d522472f72c4b5bad2237ab
Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>