makesyscalls.sh: allow pointer return types.

The previous code required that the return type be a single word. This
allows it to be a pointer without using a typedef.

Update the return types of break, mmap, and shmat to be void * as
declared. This only effects systrace output in-tree, but can aid in
generating system call wrappers from syscalls.master.

Reviewed by: kib
Obtained from: CheriBSD
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D17873