Makefile.inc1: Avoid including cwd in path

Buildworld failed when objcopy tried to overwrite itself, with `objcopy:
open objcopy failed: Text file busy`.  The PATH ended up with `::`,
effectively including the current directory, and we found the wrong
objcopy.

PR:		261215
Sponsored by:	The FreeBSD Foundation

(cherry picked from commit ac7ed26)
(cherry picked from commit e425d43)

Author: emaste

Makefile.inc1

@@ -607,7 +607,10 @@ XPATH=		${WORLDTMP}/bin:${WORLDTMP}/usr/sbin:${WORLDTMP}/usr/bin
 # When building we want to find the cross tools before the host tools in ${BPATH}.
 # We also need to add UNIVERSE_TOOLCHAIN_PATH so that we can find the shared
 # toolchain files (clang, lld, etc.) during make universe/tinderbox
-STRICTTMPPATH=	${XPATH}:${BPATH}:${UNIVERSE_TOOLCHAIN_PATH}
+STRICTTMPPATH=	${XPATH}:${BPATH}
+.if defined(UNIVERSE_TOOLCHAIN_PATH)
+STRICTTMPPATH:=${STRICTTMPPATH}:${UNIVERSE_TOOLCHAIN_PATH}
+.endif
 # We should not be using tools from /usr/bin accidentally since this could cause
 # the build to break on other systems that don't have that tool. For now we
 # still allow using the old behaviour (inheriting $PATH) if