kboot: Add HOST_O_ constants for open, etc

Add the common O_ constants for the open, fcntl, etc system calls. They
are different than FreeBSD's. While they can differ based on
architecture, they are constant for architectures we care about, and
those architectures use the 'generic' version so future architectures
will also work.

Sponsored by: Netflix

(cherry picked from commit ae366d5106844c26ecec54b6926c13064a224aa9)