Core: Increase AST_PBX_MAX_STACK to 512 if not LOW_MEMORY

The current settings AST_PBX_MAX_STACK is 128 entries which is
too low for some FreePBX installations with complex parking
arrangements.  Increased to 512 if LOW_MEMORY is not defined.

ASTERISK-28300

Change-Id: I7c4b540bc92e6642df0f3da639b003f7da8b1299