(PTR_REG): Ditto.
(PTR_SIZE): Ditto.
(PTR_LOG_SIZE): Ditto.
(start): Use GEN_DWORD to replace the .dword of HeapBase,
__bss_start__, __bss_end__, FUNCTION(_fini), env and
CommandLine; when __ILP32__ is defined, set the stack base to
the top end of the 32-bit address space if the returned value
from the Angel API call is larger than or equal to 4 GiB.
Also carry out sanity check on the heap base; abort if the
base is larger than or equal to 4 GiB. Use other new
macros in the instructions that processes the argv arrays.
(StackBase): New lable; replace __stack_base__.
(__stack_base__): Set with StackBase or StackBase + 4.
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