x86: determine MXCSR mask in all cases

x86: determine MXCSR mask in all cases

For its use(s) by the emulator to be correct in all cases, the filling
of the variable needs to be independent of XSAVE availability. As
there's no suitable function in i387.c to put the logic in, keep it in
xstate_init(), arrange for the function to be called unconditionally,
and pull the logic ahead of all return paths there.

Fixes: 9a4496a35b20 ("x86emul: support {,V}{LD,ST}MXCSR")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>