pgo-training: Pass ARCHS to the makefile

If ARCHS itself was set by the caller, it would already have been
implicitly passed to the makefile - but if the caller set
TOOLCHAIN_ARCHS instead, that wasn't acted upon by the makefile.

As the shell script already has the logic for initializing this
(in the same fashion as all other shell scripts), just use that
and pass the value on to the makefile.

pgo-training.sh

@@ -55,6 +55,7 @@ if [ ! -d $SQLITE ]; then
 fi
 
 rm -rf "$LLVM_PROFILE_DATA_DIR"
+export ARCHS
 $MAKE -f pgo-training.make PREFIX=$PREFIX STAGE1=$STAGE1 SQLITE=$SQLITE clean
 $MAKE -f pgo-training.make PREFIX=$PREFIX STAGE1=$STAGE1 SQLITE=$SQLITE -j$CORES