Bug 1719228 - Build GCC with the toolchain sysroot. r=firefox-build-system-reviewers,andi

Because GCC is built in stages, the final stage is built with
intermediate stages's GCC, which handles the sysroot correctly, so we
end up with headers and libraries with the expected compatibility.
This allows to use the same toolchain docker images as other toolchains,
based on Debian buster.

Differential Revision: https://phabricator.services.mozilla.com/D119136

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

@@ -61,7 +61,12 @@ build_gcc() {
 
   mkdir $root_dir/gcc-objdir
   pushd $root_dir/gcc-objdir
-  ../gcc-source/configure --prefix=${prefix-/tools/gcc} --build=x86_64-unknown-linux-gnu --target="${target}" --enable-languages=c,c++  --disable-nls --disable-gnu-unique-object --enable-__cxa_atexit --with-arch-32=pentiumpro --with-sysroot=/
+
+  if [ -d $MOZ_FETCHES_DIR/sysroot ]; then
+    EXTRA_CONFIGURE_FLAGS="--with-build-sysroot=$MOZ_FETCHES_DIR/sysroot"
+    export CFLAGS_FOR_BUILD="--sysroot=$MOZ_FETCHES_DIR/sysroot"
+  fi
+  ../gcc-source/configure --prefix=${prefix-/tools/gcc} --build=x86_64-unknown-linux-gnu --target="${target}" --enable-languages=c,c++  --disable-nls --disable-gnu-unique-object --enable-__cxa_atexit --with-arch-32=pentiumpro --with-sysroot=/ $EXTRA_CONFIGURE_FLAGS
   make $make_flags
   make $make_flags install DESTDIR=$root_dir
 

taskcluster/ci/toolchain/gcc.yml

@@ -5,15 +5,14 @@
 job-defaults:
     worker-type: b-linux
     worker:
-        # For now we need to keep GCC built on debian 8 because we need
-        # its headers and libraries to be compatible, as we reship them in
-        # clang.
-        docker-image: {in-tree: deb8-toolchain-build}
         max-run-time: 3600
     run:
         resources:
             - 'build/unix/build-gcc/build-gcc.sh'
         toolchain-artifact: public/build/gcc.tar.xz
+    fetches:
+        toolchain:
+            - linux64-toolchain-sysroot
 
 linux64-gcc-7:
     description: "GCC 7 toolchain build"