[Bug target/71749] Define _REENTRANT on ARC when -pthread is passed

[claziss at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71749

--- Comment #4 from Claudiu Zissulescu <claziss at gmail dot com> ---
If it is ok with you, I will do it in ur name as well. But first, let me see
how the nightly goes.