6a, 6c, 6l: fix for Plan 9 build
6a/a.h:
. Dropped <u.h> and <libc.h>.
. Made definition of EOF conditional.
6a/a.y:
. Added <u.h> and <libc.h>.
6a/lex.c:
. Added <u.h> and <libc.h>.
. Dropped <ctype.h> (now in <u.h>).
6c/gc.h:
. Added varargck pragma for "lD".
6c/swt.c:
. Dropped unused "thestring" argument in Bprint() calls.
6l/Makefile:
. Dropped unneeded directory prefix.
6l/l.h:
. Dropped unneeded directory prefix.
. Added varargck pragma for "I" and "i".
6l/obj.c:
. Dropped unneeded assignment.
. Dropped unreachable goto statement.
6l/pass.c:
. Dropped assignments flagged as unused.
6l/prof.c:
. Replaced "#if 0" with "#ifdef NOTDEF".
6l/span.c:
. Dropped unused incrementation.
. Added USED() as required.
. Dropped unreachable "return" statement.
R=golang-dev
CC=golang-dev, rsc
https://golang.org/cl/
4747044