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().
Perl_load_module() no longer moves the current stack, so no need to save it.
No need to wrap calls to Perl_load_module() in ENTER/LEAVE

As of commit 53a7735b62aee146 (May 2007) Perl_vload_module() wraps its call
to Perl_utilize() with ENTER/LEAVE, so there's no longer a need for callers
of Perl_load_module() to also wrap with ENTER/LEAVE.