makesyscalls.sh: allow pointer return types.

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