runtime: Mark runtime_goexit function as noinline.

runtime: Mark runtime_goexit function as noinline.

If the compiler inlines this function into kickoff, it may reuse
the TLS block address to load g. However, this is not necessarily
correct, as the call to g->entry in kickoff may cause the TLS
address to change. If the wrong value is loaded for g->status in
runtime_goexit, it may cause a runtime panic.

By marking the function as noinline we prevent the compiler from
reusing the TLS address.

From-SVN: r215484