Uthreads do not require TLS

By default, individual uthreads will not have their own TLS; they will piggyback
off of their vcore's TLS.  The uthread library can handle individual
uthreads having TLS.

Note that thread0 (from glibc) will have TLS, no matter what.   FYI,
thread0's TLS gets used at exit, and needs to be the same as when the
program started (C++ programs, fluidanimate at least).

2LSs choose whether or not their specific uthreads have TLS.  To be
somewhat compatible, pthreads will have them by default.  If you're app
doesn't need TLS, call pthread_need_tls(FALSE), preferably before call
pthread_create().

For reference, this lowers the latency of the context switch (via the
untuned pthread_test 100 100000 1) from ~360ns -> 194ns.  That includes
3 mcs locks in the 2LS, which we could minimize.
Removes unnecessary TLS var from vcore.c

Not sure why that was in TLS.  If I had a good reason, then I should
have left a note...
x86: catches buggy rdtscp

Not sure if this is a "bug in qemu" or something I'm doing wrong.  But rdtscp
should drop the value of MSR_TSC_AUX in ecx.  If this fails, it busts our
FAST_COREID.  If this happens to you in a VM, just change your CONFIG.

If it happens on hardware, let me know.
Vcore TLS desc in procdata (XCC)

Userspace now tells the kernel what TLS descriptor to use for each vcore.  The
main reason is that when making a new context, the kernel would zero the TLS
base, and then userspace would quickly set it to the vcore's TLS.  On amd64,
this incurs an extra syscall (for now), and on i686, it's an extra 70ns for the
lousy-LDT mangling.  This change allows the kernel to quickly set the TLS base
(and optionally not change it if it is already correct).

Note that only amd64 uses this option in the kernel; riscv and i686 still
handle this in userspace, but also expose their tlsdesc to the kernel.

The other benefit to this is debugging - it's often helpful for me to have
access to this from the kernel.

As a historic note, this isn't the first arch-dependent tidbit in procdata.
The transition_stack (vcore's stack) was added to procdata since sparc couldn't
handle not having a stack (x86 was just loading %sp in assembly before _start).