toolchain: add a version that can be bumped to force toolchain/target rebuild

This can be used to ensure that the compiled code is up to date, when
something important changes in the toolchain.
A recent example of this is the gcc 11 fix for a code miscompilation issue

Signed-off-by: Felix Fietkau <nbd@nbd.name>

.gitignore

@@ -5,6 +5,7 @@
 /dl
 /.config
 /.config.old
+/.toolchain_build_ver
 /bin
 /build_dir
 /staging_dir