Allow bootstrapping makefs on older FreeBSD hosts and Linux/macOS

In order to do so we need to install the msdosfs headers to the bootstrap
sysroot and avoid includes of kernel headers that may not exist on every
host (e.g. sys/lockmgr.h). This change should allow bootstrapping of makefs
on FreeBSD 11+ as well as Linux and macOS.

We also have to avoid using the IO_SYNC macro since that may not be
available. In makefs it is only used to switch between calling
bwrite() and bdwrite() which both call the same function. Therefore we
can simply always call bwrite().

For our CheriBSD builds we always bootstrap makefs by setting
LOCAL_XTOOL_DIRS='lib/libnetbsd usr.sbin/makefs' and use the makefs binary
from the build tree to create a bootable disk image.

Reviewed By: brooks
Differential Revision: https://reviews.freebsd.org/D23201