[AArch64] When unavailable, fetch VG from ptrace.

I was doing some SVE tests on system QEMU and noticed quite a few failures
related to inferior function calls. Any attempt to do an inferior function
call would result in the following:

Unable to set VG register.: Success.

This happens because, after an inferior function call, GDB attempts to restore
the regcache state and updates the SVE register in order. Since the Z registers
show up before the VG register, VG is still INVALID by the time the first Z
register is being updated. So when executing the following code in
aarch64_sve_set_vq:

if (reg_buf->get_register_status (AARCH64_SVE_VG_REGNUM) != REG_VALID)
  return false;

By returning false, we signal something is wrong, then we get to this:

  /* First store vector length to the thread.  This is done first to ensure the
     ptrace buffers read from the kernel are the correct size.  */
  if (!aarch64_sve_set_vq (tid, regcache))
    perror_with_name (_("Unable to set VG register."));

Ideally we'd always have a valid VG before attempting to set the Z registers,
but in this case the ordering of registers doesn't make that possible.

I considered reordering the registers to put VG before the Z registers, like
the DWARF numbering, but that would break backwards compatibility with
existing implementations. Also, the Z register numbering is pinned to the V
registers, and adding VG before Z would create a gap for non-SVE targets,
since we wouldn't be able to undefine VG for non-SVE targets.

As a compromise, it seems we can safely fetch the VG register value from
ptrace. The value in the kernel is likely the updated value anyway.

This patch fixed all the failures i saw in the testsuite and caused no further
regressions.

gdb/ChangeLog:

2020-03-19  Luis Machado  <luis.machado@linaro.org>

	* nat/aarch64-sve-linux-ptrace.c (aarch64_sve_set_vq): If vg is not
	valid, fetch vg value from ptrace.