libgcc: Honor LDFLAGS_FOR_TARGET when linking libgcc_s

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-darwin (SHLIB_LINK): Likewise.
	* config/t-slibgcc-vms (SHLIB_LINK): Likewise.
	* config/t-slibgcc-fuchsia (SHLIB_LDFLAGS): Remove $(LDFLAGS).