sys: Declare 'end' as an extern char so that &end is a char *

[jhb]

Summary

While here, remove an unused declaration.

Sponsored by: AFRL, DARPA

[jrtc27]

s/extern char/&[]/ in the subject, also &end is not char * in that case, it's char (*)[], which is an incomplete type so you can't perform pointer arithmetic on it (as sizeof(char[]) is not known). end itself decays to a char * and so pointer arithmetic on *that* works (or &end[0], but decaying is cleaner). In this case though there's an explicit cast before the arithmetic so both "work".