Fix aarch64-freebsd build failure

Fix aarch64-freebsd build failure

I happen to see that fbsd-tdep.o is missing for target aarch64-freebsd,
and it causes the build failure,

aarch64-fbsd-tdep.o: In function `aarch64_fbsd_init_abi(gdbarch_info, gdbarch*)':
binutils-gdb/gdb/aarch64-fbsd-tdep.c:186: undefined reference to `fbsd_init_abi(gdbarch_info, gdbarch*)'
binutils-gdb/gdb/aarch64-fbsd-tdep.c:189: undefined reference to `svr4_lp64_fetch_link_map_offsets()'
binutils-gdb/gdb/aarch64-fbsd-tdep.c:189: undefined reference to `set_solib_svr4_fetch_link_map_offsets(gdbarch*, link_map_offsets* (*)())'

This patch fixed it.

gdb:

2017-09-21  Yao Qi  <yao.qi@linaro.org>

	* configure.tgt (aarch64*-*-freebsd*): Add fbsd-tdep.o solib-svr4.o
	to gdb_target_obs.