Add GC root for func in jl_instantiate_staged

[timholy]

Fixes #9099, I hope. CC @Keno, @JeffBezanson, @vtjnash. With this change, I was able to insert a jl_gc_collect() here without causing a segfault.

[vtjnash]

lgtm. otherwise the first argument to jl_apply isn't rooted

[timholy]

Thanks, @vtjnash. This is my first time using GC_PUSH macros, so I greatly appreciate your having taken the time to look at this. Am I right in thinking that all functions with jl_ in front of them might involve allocation, and hence you need to root anything they use? And conversely, in the gaps between jl_* functions you should not require gc roots?

[coveralls]

Changes Unknown when pulling 6fcbf1b on teh/staged_root into * on master*.