Move zfs_gitrev.h to build directory

Currently an out-of-tree build does not work with read-only source
directory because zfs_gitrev.h can't be created. Move this file to the
build directory, which is more appropriate for a generated file, and
drop the dist-hook for zfs_gitrev.h. There is no need to distribute this
file since it will be regenerated as part of the compilation in any
case.

scripts/make_gitrev.sh tries to avoid updating zfs_gitrev.h if there has
been no change, however this doesn't cover the case when the source
directory is not in git: in that case zfs_gitrev.h gets overwritten even
though it's always "unknown". Simplify the logic to always write out a
new version of zfs_gitrev.h, compare against the old and overwrite only
if different. This is now simple enough to just include in the
Makefile, so drop the script.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Closes #10493