x86: Fix compiler warning

arch/x86/crtools.c: In function ‘arch_alloc_thread_info’:
arch/x86/crtools.c:271:6: error: ‘with_xsave’ may be used uninitialized in this function [-Werror=uninitialized]

Actually the with_xsave is with_fpu dependant, but some gccs
can't guess that fact :\

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>