Use pulongest in aarch64-linux-tdep.c

Use pulongest in aarch64-linux-tdep.c

While testing a patch on the buildbot, I got this error:

../../binutils-gdb/gdb/aarch64-linux-tdep.c: In function uint64_t aarch64_linux_core_read_vq(gdbarch*, bfd*):
../../binutils-gdb/gdb/aarch64-linux-tdep.c:285:29: error: format %ld expects argument of type long int, but argument 2 has type uint64_t {aka long long unsigned int} [-Werror=format=]

This patch avoids the problem by using pulongest rather than %ld.
This seems safe to me because, if aarch64-linux-tdep.c is included in
the build, then ULONGEST must be a 64-bit type.

gdb/ChangeLog
2018-08-15  Tom Tromey  <tom@tromey.com>

	* aarch64-linux-tdep.c (aarch64_linux_core_read_vq): Use pulongest.