x86: Fix initialization of new threads

fninit does not clear MMX/SSE/AVX registers, so we have to use rstor to
clear them when starting a new thread. Along the way, we can as well
just have a default state to be loaded in each new thread.

* i386/include/mach/i386/fp_reg.h (XSAVE_XCOMP_BV_COMPACT): New macro.
* i386/i386/fpu.h (fp_default_state): New variable declaration.
* i386/i386/fpu.c (fp_default_state): New variable.
(MXCSR_DEFAULT, CWD_DEFAULT): New macros.
(fpu_module_init): Allocate and initialize fp_default_state.
(fpinit): rstor from fp_default_state instead of setting FPU state by
hand.
(fp_load): Copy initial state from fp_default_state instead of setting
it to 0. This is more future-proof since this is the exact state that we are
loading in fpinit.
(fp_state_alloc): fp_state_alloc: Copy initial state from
fp_default_state.

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