don't suppress shared libc when linker lacks -Bsymbolic-functions

previous work overhauling the dynamic linker made it so that linking
libc with -Bsymbolic-functions was no longer mandatory, but the
configure logic that forced --disable-shared when ld failed to accept
the option was left in place.

this commit removes the hard-coded -Bsymbolic-functions from the
Makefile and changes the configure test to one that simply adds it to
the auto-detected LDFLAGS on success.
diff --git a/Makefile b/Makefile
index 7c3dda4..b2a719d 100644
--- a/Makefile
+++ b/Makefile
@@ -159,8 +159,7 @@
 
 lib/libc.so: $(LOBJS) $(LDSO_OBJS)
 	$(CC) $(CFLAGS_ALL) $(LDFLAGS_ALL) -nostdlib -shared \
-	-Wl,-e,_dlstart -Wl,-Bsymbolic-functions \
-	-o $@ $(LOBJS) $(LDSO_OBJS) $(LIBCC)
+	-Wl,-e,_dlstart -o $@ $(LOBJS) $(LDSO_OBJS) $(LIBCC)
 
 lib/libc.a: $(AOBJS)
 	rm -f $@