Apply mapping protections to .o kernel modules.

Use the section flags to derive mapping protections. When multiple
sections overlap within a page, the union of their protections must be
applied. With r353701 the .text and .rodata sections are padded to
ensure that this does not happen on amd64.

Reviewed by: kib
MFC after: 1 month
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D21896