Fix GDB build fail on Aarch64 when -fno-common is enabled

Fix GDB build fail on Aarch64 when -fno-common is enabled

Current trunk GDB (and gdb-7.8.1 too) fails to build on Aarch64 when
-fno-common is enabled.  It fails during link stage due to multiple
definition of `tdesc_aarch64':

...
[  199s] aarch64-linux-nat.o: In function `initialize_tdesc_aarch64':
[  199s]
/home/abuild/rpmbuild/BUILD/gdb-7.8.1/gdb/features/aarch64.c:11:
multiple definition of `tdesc_aarch64'
[  199s]
aarch64-tdep.o:/home/abuild/rpmbuild/BUILD/gdb-7.8.1/gdb/objfiles.h:540:
first defined here
[  199s] aarch64-linux-nat.o: In function `initialize_tdesc_aarch64':
[  199s]
/home/abuild/rpmbuild/BUILD/gdb-7.8.1/gdb/features/aarch64.c:11:
multiple definition of `tdesc_aarch64'
[  199s]
aarch64-tdep.o:/home/abuild/rpmbuild/BUILD/gdb-7.8.1/gdb/objfiles.h:540:
first defined here
[  199s] collect2: error: ld returned 1 exit status
[  199s] make[2]: *** [gdb] Error 1
...

This happens because struct target_desc *tdesc_aarch64 is defined in
gdb/features/aarch64.c, which is included by two files
(gdb/aarch64-linux-nat.c and gdb/aarch64-tdep.c).

gdb/Changelog
2015-02-17  Max Ostapenko  <m.ostapenko@partner.samsung.com>

	PR gdb/17984
	* aarch64-linux-nat.c: Don't include features/aarch64.c anymore.
	(aarch64_linux_read_description): Remove initialize_tdesc_aarch64
	call.
	* aarch64-tdep.h (tdesc_aarch64): Declare.