Use -fno-asynchronous-unwind-tables to compile lib/csu

Summary:
In r209294 kib added -fno-asynchronous-unwind-tables to the compile
flags for the GNU C startup components. This was done to work around a
BFD ld assertion, "no .eh_frame_hdr table will be created", which is
produced because of the layout of the startup objects.

Add the same flag to lib/csu too, for the same reason. And similarly to
r209294, also add -fno-omit-frame-pointer.

This is primarily meant to quickly MFC to stable/11, so it can end up in
the 11.4 release, as a fix for https://bugs.freebsd.org/246322.

PR:		246322
MFC after:	3 days
Differential Revision: https://reviews.freebsd.org/D24797

lib/csu/Makefile.inc

@@ -13,6 +13,9 @@ NO_WMISSING_VARIABLE_DECLARATIONS=
 OBJS+=	crtbegin.o crtbeginS.o crtbeginT.o
 OBJS+=	crtend.o crtendS.o
 
+CFLAGS+=	-fno-asynchronous-unwind-tables
+CFLAGS+=	-fno-omit-frame-pointer
+
 CFLAGS_CRTS=	-DSHARED ${PICFLAG}
 
 crtbegin.o: crtbegin.c