gdbserver: Add a function to set the XSAVE mask and size.

gdbserver: Add a function to set the XSAVE mask and size.

Make x86_xcr0 private to i387-fp.cc and use i387_set_xsave_mask to set
the value instead.  Add a static global instance of x86_xsave_layout
and initialize it in the new function as well to be used in a future
commit to parse XSAVE extended state regions.

Update the Linux port to use this function rather than setting
x86_xcr0 directly.  In the case that XML is not supported, don't
bother setting x86_xcr0 to the default value but just omit the call to
i387_set_xsave_mask as i387-fp.cc defaults to the SSE case used for
non-XML.

In addition, use x86_xsave_length to determine the size of the XSAVE
register set via CPUID.

Approved-By: Simon Marchi <simon.marchi@efficios.com>