S_process_special_blocks() should use a new stack for BEGIN blocks.
This avoids the stack moving underneath anything that directly or indirectly
calls Perl_load_module().
[Committer’s note:
This fixes bug #119993.
Furthermore, under STRESS_REALLOC, t/io/layers.t was crashing like this:
$ ./perl -Ilib -e ' open(UTF, "<:raw:encoding(utf8)", 'tmp75851B') or die $!; ref #blahblahblahblahblahblahblahblahblah'
Segmentation fault: 11
(The comment seems to be necessary to make it crash.)
It was happening because open() was causing a module to be loaded
while the arguments to open() were still on the stack.
]