libgcc: Honor LDFLAGS_FOR_TARGET when linking libgcc_s

When building gcc with some specific LDFLAGS_FOR_TARGET, e.g.
LDFLAGS_FOR_TARGET=-Wl,-z,relro,-z,now
those flags propagate info linking of target shared libraries,
e.g. lib{ubsan,tsan,stdc++,quadmath,objc,lsan,itm,gphobos,gdruntime,gomp,go,gfortran,atomic,asan}.so.*
but there is one important exception, libgcc_s.so.* linking ignores it.

The following patch fixes that.

Bootstrapped/regtested on x86_64-linux with LDFLAGS_FOR_TARGET=-Wl,-z,relro,-z,now
and verified that libgcc_s.so.* is BIND_NOW when it previously wasn't, and
without any LDFLAGS_FOR_TARGET on x86_64-linux and i686-linux.
There on x86_64-linux I've verified that the libgcc_s.so.1 linking command
line for -m64 is identical except for whitespace to one without the patch,
and for -m32 multilib $(LDFLAGS) actually do supply there an extra -m32
that also repeats later in the @multilib_flags@, which should be harmless.

2021-08-04  Jakub Jelinek  <jakub@redhat.com>

	* config/t-slibgcc (SHLIB_LINK): Add $(LDFLAGS).
	* config/t-slibgcc-vms (SHLIB_LINK): Likewise.

--- libgcc/config/t-slibgcc
+++ libgcc/config/t-slibgcc
@@ -32,7 +32,7 @@ SHLIB_INSTALL_SOLINK = $(LN_S) $(SHLIB_SONAME) \
 	$(DESTDIR)$(slibdir)$(SHLIB_SLIBDIR_QUAL)/$(SHLIB_SOLINK)
 
 SHLIB_LINK = $(CC) $(LIBGCC2_CFLAGS) -shared -nodefaultlibs \
-	$(SHLIB_LDFLAGS) \
+	$(SHLIB_LDFLAGS) $(LDFLAGS) \
 	-o $(SHLIB_DIR)/$(SHLIB_SONAME).tmp @multilib_flags@ \
 	$(SHLIB_OBJS) $(SHLIB_LC) && \
 	rm -f $(SHLIB_DIR)/$(SHLIB_SOLINK) && \
--- libgcc/config/t-slibgcc-vms
+++ libgcc/config/t-slibgcc-vms
@@ -22,7 +22,7 @@ SHLIB_LINK = \
   objdump --syms $(SHLIB_OBJS) | \
   $(SHLIB_SYMVEC) >> SYMVEC_$$$$.opt ; \
   echo "case_sensitive=NO" >> SYMVEC_$$$$.opt; \
-  $(CC) $(LIBGCC2_CFLAGS) -nodefaultlibs \
+  $(CC) $(LIBGCC2_CFLAGS) $(LDFLAGS) -nodefaultlibs \
   -shared --for-linker=/noinform -o $(SHLIB_NAME) $(SHLIB_OBJS) \
   --for-linker=SYMVEC_$$$$.opt \
   --for-linker=gsmatch=equal,$(shlib_version)