gcc-as-needed.diff patch broke mpx support in GCC

[Alexander Ivchenko <aivchenk@gmail.com>]

To: debian-gcc@lists.debian.org
From: aivchenk@gmail.com
Subject: gcc-as-needed.diff patch broke mpx support in GCC

Package: gcc-5
Version: gcc (Ubuntu 5.4.0-6ubuntu1~16.04.2) 5.4.0 20160609

gcc-5 from Ubuntu is configured with MPX support, but it is broken due
to always-added linker option "-as-needed".

This happens because usual invokation of gcc for mpx (e.g gcc
-fcheck-pointer-bounds -mmpx hello.c) would implicitely add -lmpx and
-lmpxwrappers during linking,
but those libraries are not actually linked:

> ldd a.out
        linux-vdso.so.1 =>  (0x00007ffe725af000)
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f3ed2513000)
        /lib64/ld-linux-x86-64.so.2 (0x000055922a9eb000)
(if we add -Wl,--no-as-needed:
> ldd a.out
        linux-vdso.so.1 =>  (0x00007ffe201e5000)
        libmpx.so.0 => /usr/lib/x86_64-linux-gnu/libmpx.so.0
(0x00007f9e76a76000)
        libmpxwrappers.so.0 =>
/usr/lib/x86_64-linux-gnu/libmpxwrappers.so.0 (0x00007f9e76873000)
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f9e764a9000)
        libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0
(0x00007f9e7628c000)
        /lib64/ld-linux-x86-64.so.2 (0x000055f6e450d000))

The fix for that would be to add the same guards for as-needed as for
sanitizer libraries