gdb/x86: Fix write out of mxcsr register for xsave targets

gdb/x86: Fix write out of mxcsr register for xsave targets

In commit:

  commit 8ee22052f690c007556b97eed59f49350ece5ca9
  Author: Andrew Burgess <andrew.burgess@embecosm.com>
  Date:   Thu May 3 17:46:14 2018 +0100

      gdb/x86: Handle kernels using compact xsave format

in two places FXSAVE_ADDR was used instead of FXSAVE_MXCSR_ADDR to get
the address of the mxcsr register within the xsave buffer.  This will
mean we are potentially accessing the wrong location within the xsave
buffer.

There are no tests included with this patch.  The first mistake would
only trigger an issue if/when the user tries to manually set the mxcsr
register to a value that matches the random (value off stack) value
that is in the xsave buffer, in this case the change by the user will
go unnoticed by GDB, and the default value of mxcsr will be preserved.

The second mistake only happens on the code path where all x87
registers are being written out of the register cache.  I'm not sure
how to trigger that code path.

gdb/ChangeLog:

	* i387-tdep.c (i387_collect_xsave): Use FXSAVE_MXCSR_ADDR not
	FXSAVE_ADDR for the mxcsr register.