Add -fno-omit-frame-pointer to CFLAGS used to compile crt1.c on amd64.

For gcc' __builtin_frame_address() to work, all call frames need to save
frame pointer. In particular, this is important for the upper frame that
should terminate the chain.

No objections from:	jhb
PR:	amd64/126543
MFC after:	1 week

lib/csu/amd64/Makefile

@@ -7,6 +7,7 @@ OBJS=		${SRCS:N*.h:R:S/$/.o/g}
 OBJS+=		gcrt1.o
 CFLAGS+=	-I${.CURDIR}/../common \
 		-I${.CURDIR}/../../libc/include
+CFLAGS+=	-fno-omit-frame-pointer
 
 all: ${OBJS}