makesyscalls.sh: always use absolute path for syscalls.conf
syscalls.conf is included using "." which per the Open Group:
If file does not contain a <slash>, the shell shall use the search
path specified by PATH to find the directory containing file.
POSIX shells don't fall back to the current working directory.
Submitted by: Nathaniel Wesley Filardo <firstname.lastname@example.org>
Reviewed by: bdrewery
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D20476