Return X86_XSTATE_SSE_MASK instead of 0 in i386fbsd_core_read_xcr0

Return X86_XSTATE_SSE_MASK instead of 0 in i386fbsd_core_read_xcr0

i386fbsd_core_read_xcr0 reads the value of xcr0 from the corefile.  If
it fails, returns 0.  This makes its caller {i386,amd64}_target_description
has to handle this special value.  IMO, i386fbsd_core_read_xcr0 should
return the default xcr0 in case of error.

gdb:

2017-09-04  Yao Qi  <yao.qi@linaro.org>

	* i386-fbsd-tdep.c (i386fbsd_core_read_xcr0): Return
	X86_XSTATE_SSE_MASK instead of 0.