For amd64, explicitly compile mcount.po, rather than copying mcount.o. We

need to compile it with -fno-omit-frame-pointers since the mcount code
depends on that, and by default it omits them without -pg.

lib/libc/gmon/Makefile.inc

@@ -10,6 +10,12 @@ MAN+=	moncontrol.3
 
 MLINKS+=moncontrol.3 monstartup.3
 
+.if ${MACHINE_ARCH} == amd64
+# mcount needs to be compiled with frame pointers and without profiling
+mcount.po: mcount.c
+	${CC} ${CFLAGS} -fno-omit-frame-pointer -c ${.IMPSRC} -o ${.TARGET}
+.else
 # mcount cannot be compiled with profiling
 mcount.po: mcount.o
 	cp mcount.o mcount.po
+.endif