makesyscalls.lua: trim trailing spaces/commas from args

These are insignificant as far as declarations go, and we've historically
allowed it. fhlinkat in ^/sys/kern/syscalls.master, for example, currently
has a trailing comma after its final argument that this version of
makesyscalls is ignoring (not by conscious decision).

Fix it for now by actively stripping off trailing whitespace/commas until
we decide to actively prohibit it.