Abandon use of transition stacks

They are no longer necessary in this implementation.
Rewrite __vcore_reenter in assembly

This ensures legal stack discipline.  The compiler cannot be trusted to
generate correct code when the stack pointer changes under its feet.

We needed to allocate two words at the top of the stack to avoid a SIGSEGV
in google-pprof.  Not exactly sure why, but this fixes it.
Prevent vcore_test from locking up randomly

The test was simply incorrect.
Don't bail out if setaffinity fails

This allows us to use VCORE_LIMIT > get_nprocs() for debugging.
Refactor TLS initialization

IA-32 requires us to initialize the TLS before setting the TLS descriptor.
The previous code structure did them in the opposite order.
Add memory allocation helpers