unix/gcollect: Make sure stack/regs get captured properly for GC.

When building with link time optimization enabled it is possible both
gc_collect() and gc_collect_regs_and_stack() get inlined into gc_alloc()
which can result in the regs variable being pushed on the stack earlier
than some of the registers. Depending on the calling convention, those
registers might however contain pointers to blocks which have just been
allocated in the caller of gc_alloc(). Then those pointers end up higher on
the stack than regs, aren't marked by gc_collect_root() and hence get
sweeped, even though they're still in use.

As reported in #4652 this happened for in 32-bit msvc release builds:
mp_lexer_new() does two consecutive allocations and the latter triggered a
gc_collect() which would sweep the memory of the first allocation again.