Inline RB_VM_SAVE_MACHINE_CONTEXT into BLOCKING_REGIONHEADmaster

There's an exhaustive explanation of this in the linked redmine bug, but
the short version is as follows:

blocking_region_begin can spill callee-saved registers to the stack for
its own use. That means they're not saved to ec->machine by the call to
setjmp, since by that point they're already on the stack and new,
different values are in the real registers. ec->machine's end-of-stack
pointer is also bumped to accomodate this, BUT, after
blocking_region_begin returns, that points past the end of the stack!

As far as C is concerned, that's fine; the callee-saved registers are
restored when blocking_region_begin returns. But, if another thread
triggers GC, it is relying on finding references to Ruby objects by
walking the stack region pointed to by ec->machine.

If the C code in exec; subsequently does things that use that stack
memory, then the value will be overwritten and the GC might prematurely
collect something it shouldn't.

[Bug #20493]

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