This commit changes how stack extents are calculated for both the main
thread and other threads. Ruby uses the address of a local variable as
part of the calculation for machine stack extents:

* pthreads uses it as a lower-bound on the start of the stack, because
  glibc (and maybe other libcs) can store its own data on the stack
  before calling into user code on thread creation.
* win32 uses it as an argument to VirtualQuery, which gets the extent of
  the memory mapping which contains the variable

However, the local being used for this is actually too low (too close to
the leaf function call) in both the main thread case and the new thread
case.

In the main thread case, we have the `INIT_STACK` macro, which is used
for pthreads to set the `native_main_thread->stack_start` value. This
value is correctly captured at the very top level of the program (in
main.c). However, this is _not_ what's used to set the execution context
machine stack (`th->ec->machine_stack.stack_start`); that gets set as
part of a call to `ruby_thread_init_stack` in `Init_BareVM`, using the
address of a local variable allocated _inside_ `Init_BareVM`. This is
too low; we need to use a local allocated closer to the top of the
program.

In the new thread case, the lolcal is allocated inside
`native_thread_init_stack`, which is, again, too low.

In both cases, this means that we might have VALUEs lying outside the
bounds of `th->ec->machine.stack_{start,end}`, which won't be marked
correctly by the GC machinery.

To fix this,

* In the main thread case: We already have `INIT_STACK` at the right
  level, so just pass that local var to `ruby_thread_init_stack`.
* In the new thread case: Allocate the local one level above the call to
  `native_thread_init_stack` in `call_thread_start_func2`.

[Bug #20001]

fix