Bug 1427339 - Configure binutils and gcc --with-sysroot=/. r=gps

The system binutils and gcc are built with that option on Debian, but
not on CentOS. That makes no practical difference, except for the fact
that when building GCC, we use our own-built binutils (as per bug
1427316), but use the system GCC. And a GCC built with --with-sysroot=/
doesn't work with a binutils built without. However, a GCC built without
--with-sysroot=/ works fine with a binutils built with it. So this
change is compatible with building our GCC on both CentOS and Debian.

build/unix/build-gcc/build-gcc.sh

@@ -97,7 +97,7 @@ build_binutils() {
   then
     # gold is disabled because we don't use it on automation, and also we ran into
     # some issues with it using this script in build-clang.py.
-    binutils_configure_flags="--disable-gold --enable-plugins --disable-nls"
+    binutils_configure_flags="--disable-gold --enable-plugins --disable-nls --with-sysroot=/"
   fi
 
   mkdir $root_dir/binutils-objdir
@@ -112,7 +112,7 @@ build_binutils() {
 build_gcc() {
   mkdir $root_dir/gcc-objdir
   pushd $root_dir/gcc-objdir
-  ../gcc-$gcc_version/configure --prefix=${prefix-/tools/gcc} --enable-languages=c,c++  --disable-nls --disable-gnu-unique-object --enable-__cxa_atexit --with-arch-32=pentiumpro --disable-initfini-array
+  ../gcc-$gcc_version/configure --prefix=${prefix-/tools/gcc} --enable-languages=c,c++  --disable-nls --disable-gnu-unique-object --enable-__cxa_atexit --with-arch-32=pentiumpro --disable-initfini-array --with-sysroot=/
   make $make_flags
   make $make_flags install DESTDIR=$root_dir