Could not take a signal on threads other than the main thread.
If you look at the spinning binary with dtrace, you can see a
fault happening over and over:

    $ dtrace -n '
    fbt::user_trap:entry /execname=="boot32" && self->count < 10/
    {
        self->count++;
        printf("%s %x %x %x %x", probefunc, arg1, arg2, arg3, arg4);
        stack();
        tracemem(arg4, 256);
    }'

    dtrace: description 'fbt::user_trap:entry ' matched 1 probe
    CPU     ID                    FUNCTION:NAME
      1  17015                  user_trap:entry user_trap 0 10 79af0a0 79af0a0
                  mach_kernel`lo_alltraps+0x12a

             0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f  0123456789abcdef
         0: 0e 00 00 00 37 00 00 00 00 00 00 00 1f 00 00 00  ....7...........
        10: 1f 00 00 00 a8 33 00 00 00 00 00 01 00 00 00 00  .....3..........
        20: 98 ba dc fe 07 09 00 00 00 00 00 00 98 ba dc fe  ................
        30: 06 00 00 00 0d 00 00 00 34 00 00 00 9e 1c 00 00  ........4.......
        40: 17 00 00 00 00 02 00 00 ac 30 00 00 1f 00 00 00  .........0......
        50: 00 00 00 00 00 00 00 00 0d 00 00 00 e0 e6 29 00  ..............).
        60: 34 00 00 00 00 00 00 00 9e 1c 00 00 00 00 00 00  4...............
        70: 17 00 00 00 00 00 00 00 00 02 00 00 00 00 00 00  ................
        80: ac 30 00 00 00 00 00 00 1f 00 00 00 00 00 00 00  .0..............
        90: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
        a0: 48 00 00 00 10 00 00 00 85 00 00 00 a0 f2 29 00  H.............).
        b0: 69 01 00 02 00 00 00 00 e6 93 04 82 ff 7f 00 00  i...............
        c0: 2f 00 00 00 00 00 00 00 06 02 00 00 00 00 00 00  /...............
        d0: 78 ee 42 01 01 00 00 00 1f 00 00 00 00 00 00 00  x.B.............
        e0: 00 ed 9a 07 00 00 00 00 00 00 00 00 00 00 00 00  ................
        f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................

    ...

The memory dump shows a 32-bit exception frame:

    x86_saved_state32

    gs = 0x37
    fs = 0
    es = 0x1f
    ds = 0x1f
    edi = 0x33a8
    esi = 0x01000000
    ebp = 0
    cr2 = 0xfedcba98
    ebx = 0x0907
    edx = 0
    ecx = 0xfedcba98
    eax = 0x06
    trapno = 0x0d
    err = 0x34
    eip = 0x1c9e
    cs = 0x17
    efl = 0x0200
    uesp = 0x30ac
    ss = 0x1f

The cr2 of 0xfedcba98 is the address that the new thread read
to cause the fault, but note that the trap is now a GP fault with
error code 0x34, meaning it's moved past the cr2 problem and on
to an invaild segment selector.  The 0x34 is suspiciously similar
to the 0x37 in gs, and sure enough, OS X forces gs to have
that value in the signal handler, and if your thread hasn't set
up that segment (known as USER_CTHREAD), you'll fault on the IRET
into the signal handler and never be able to handle a signal.

The kernel bug is that it forces segment 0x37 without making sure
it is a valid segment.  Leopard also forced 0x37 but had the courtesy
to set it up first.

Since OS X requires us to set up that segment (using the
thread_fast_set_cthread_self system call), we might as well
use it instead of the more complicated i386_set_ldt call to
set up our per-OS thread storage.

Also add some more zeros to bsdthread_register for new arguments
in Snow Leopard (apparently unnecessary, but being careful).

Fixes #510.

R=r
CC=golang-dev
https://golang.org/cl/824046