Add a .PATH entry to search for sources in the ${MACHINE}/${MACHINE}

directory before the ${MACHINE_ARCH}/${MACHINE_ARCH} directory so that
machine-specific files take precedence of architecture-specific ones.

This fixes the build on sun4v which doesn't use the sparc64 version
of mem.c.

Tested by: make universe

sys/modules/mem/Makefile

@@ -1,6 +1,7 @@
 # $FreeBSD$
 
 .PATH: ${.CURDIR}/../../dev/mem
+.PATH: ${.CURDIR}/../../${MACHINE}/${MACHINE}
 .PATH: ${.CURDIR}/../../${MACHINE_ARCH}/${MACHINE_ARCH}
 
 KMOD=	mem