Use gdb::byte_vector when reading section data

Use gdb::byte_vector when reading section data

This changes a couple of spots that read section data to use
gdb::byte_vector rather than a cleanup.

Regression tested by the buildbot.  I am not certain that the buildbot
actually tests the code in question, so I recommend careful review.

gdb/ChangeLog
2018-03-12  Tom Tromey  <tom@tromey.com>

	* rs6000-aix-tdep.c (rs6000_aix_core_xfer_shared_libraries_aix):
	Use gdb::byte_vector.
	* arm-tdep.c (arm_exidx_new_objfile): Use gdb::byte_vector.