Merge GDB 7.6.50

Pulled in from:
ftp://sourceware.org/pub/gdb/snapshots/current/gdb-7.6.50.20131211.tar.bz2

Fixes included:
- reorder lines to move declarations to top of functions in gdb/amd-tdep.c
- new header includes in nacl-manifest.c
- target_gdbarch uses changed to target_gdbarch() as it's now a function
pointer

BUG=None
TEST=gdb_tests + debug_stub_tests
R=halyavin@google.com, eaeltsin@google.com, mcgrathr@chromium.org