S_process_special_blocks() should use a new stack for BEGIN blocks.

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.
]