Fix for SF bug #576327: zipfile when sizeof(long) == 8

Fix for SF bug #576327: zipfile when sizeof(long) == 8
binascii_crc32():  Make this return a signed 4-byte result across
platforms.  The other way to make this platform-independent would be to
make it return an unsigned unbounded int, but the evidence suggests
other code out there treats it like a signed 4-byte int (e.g., existing
code writing the result with struct.pack "l" format).

Bugfix candidate.