Amend POSIX errno test to use new opcodes

We're now using nqp::x_posixerrno for retrieving the value,
and pir::const::POSIX_* for the values of the constants
Add script to generate list of errno value macros
Generate posix.pasm as dependency for building NQP
Include posix.pasm in registered constants
Remove nqp::posixerrnovalues, and rename nqp::posixerrno
Ignore generated posix.pasm file