Hi,

current trunk GDB (and gdb-7.8.1 too) fails to build on Aarch64 when 
-fno-common is enabled with:

progspace.o skip.o probe.o common-utils.o buffer.o ptid.o gdb-dlfcn.o 
common-agent.o format.o registry.o btrace.o record-btrace.o waitstatus.o 
print-utils.o rsp-low.o errors.o common-debug.o debug.o 
common-exceptions.o btrace-common.o compile.o compile-c-symbols.o 
compile-c-types.o compile-object-load.o compile-object-run.o 
compile-loc2c.o compile-c-support.o inflow.o init.o \
[  190s] >------   ../readline/libreadline.a ../opcodes/libopcodes.a 
../bfd/libbfd.a  ../libiberty/libiberty.a 
../libdecnumber/libdecnumber.a    -ldl -ldl -lncurses -lm -ldl -lpthread 
-ldl -lutil -lm -lpython2.7 -Xlinker -export-dynamic 
/usr/lib64/libexpat.so   ../libiberty/libiberty.a 
build-gnulib/import/libgnu.a -ldl -Wl,--dynamic-list=./proc-service.list
[  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
[  199s] make[2]: Leaving directory 
`/home/abuild/rpmbuild/BUILD/gdb-7.8.1/gdb'
[  199s] make[1]: *** [all-gdb] Error 2
[  199s] make[1]: Leaving directory `/home/abuild/rpmbuild/BUILD/gdb-7.8.1'
[  199s] make: *** [all] Error 2

This happens because struct target_desc *tdesc_aarch64 is defined in 
gdb/features/aarch64.c source file, that is included to some other files
(gdb/aarch64-linux-nat.c, for example). So, we have multiple definition 
of `tdesc_aarch64' error during link stage.

This small patch resolves the issue, OK to commit?

-Maxim