On macOS, add 64 bytes of headerpad to accommodate code signatures

[geofft]

Closes #749.

[geofft]

Failing tests from previous run, as expected on x86_64 only:

Error: errors found
  error: python/install/lib/python3.13/lib-dynload/_ctypes_test.cpython-313d-darwin.so: Insufficient headerpad between end of load commands 0x648 and beginning of code 0x650
  error: python/install/lib/python3.13/lib-dynload/_tkinter.cpython-313d-darwin.so: Insufficient headerpad between end of load commands 0x658 and beginning of code 0x660

[indygreg]

This looks good.

What's also surprising about this is that usually segments are aligned on 4k or 16k boundaries. i.e. there's usually a significant gap between load commands and the next data.

And load commands are usually nowhere close to 4k in size.

Again, I'm surprised there isn't a sufficient gap for the code signature load command.

But surprised or not, having a static check to ensure existence of a gap keeps us honest and is necessary.