Log Message:
OpenBSD and derivatives define MACHINE_ARCH in sys/param.h to "amd64" which
overrides our attempt to set it to "x86_64" and ensure consistency across
platforms.  Work around this by setting it using PKGSRC_MACHINE_ARCH.

Confirmed to fix the issue and not break a variety of other platforms by
Sevan, and approved under duress by Joerg.