external_deps: rebuild nacl_loader for Linux amd64

Add the naclruntime target to external_deps/build.sh. It builds
nacl_loader and nacl_helper_bootstrap.
This uses our version of the native_client repository with a hacked
build script that lets us build native binaries without Chromium tools.
The build brings two dependencies: LLVM and SCons.

Fixes #1504 (NaCl crash handling doesn't work with recent Linux
kernels.)

Author: slipher

external_deps/build.sh

@@ -889,6 +889,34 @@ build_naclports() {
 	cp pepper_*"/ports/lib/newlib_pnacl/Release/"{liblua.a,libfreetype.a,libpng16.a} "${PREFIX}/pnacl_deps/lib"
 }
 
+
+NATIVECLIENT_REVISION=27ccec9ff58b04b7635cd8a7b5b997a58c071952
+# Only builds nacl_loader and nacl_helper_bootstrap for now, not IRT.
+build_naclruntime() {
+	case "${PLATFORM}" in
+	linux-amd64-default)
+		local NACL_ARCH=x86-64
+		;;
+	*)
+		log error "naclruntime not implemented for ${PLATFORM}"
+		;;
+	esac
+
+	local dir_name="DaemonEngine-native_client-${NATIVECLIENT_REVISION:0:7}"
+	local archive_name="native_client-${NATIVECLIENT_REVISION}.zip"
+
+	download_extract naclruntime "${archive_name}" \
+		"https://api.github.com/repos/DaemonEngine/native_client/zipball/${NATIVECLIENT_REVISION}"
+
+	"${download_only}" && return
+
+	cd "${dir_name}"
+	scons "platform=${NACL_ARCH}" MODE=opt-linux werror=0 naclsdk_validate=0 sysinfo=0 sel_ldr
+	cp "scons-out/opt-linux-${NACL_ARCH}/staging/nacl_helper_bootstrap" "${PREFIX}/nacl_helper_bootstrap"
+	cp "scons-out/opt-linux-${NACL_ARCH}/staging/sel_ldr" "${PREFIX}/nacl_loader"
+}
+
+
 # The import libraries generated by MinGW seem to have issues, so we use LLVM's version instead.
 # So LLVM must be installed, e.g. 'sudo apt install llvm'
 build_genlib() {
@@ -1265,6 +1293,7 @@ fi
 
 # Enable parallel build
 export MAKEFLAGS="-j`nproc 2> /dev/null || sysctl -n hw.ncpu 2> /dev/null || echo 1`"
+export SCONSFLAGS="${MAKEFLAGS}"
 
 # Setup platform
 platform="${1}"; shift