[x86/Linux] Fix the mistmatch between gc_thread_stub and GCThreadFunction

[parjong]

This commit fixes the following warning during x86/Linux build:

/home/parjong/projects/dotnet/coreclr/src/gc/gc.cpp:5192:42: error: cannot initialize a parameter of type 'GCThreadFunction' (aka 'void (*)(void *)') with an lvalue of type 'void (void *) __attribute__((stdcall))'
    return GCToOSInterface::CreateThread(gc_thread_stub, this, &affinity);
                                         ^~~~~~~~~~~~~~
/home/parjong/projects/dotnet/coreclr/src/gc/env/gcenv.os.h:153:47: note: passing argument to parameter 'function' here
    static bool CreateThread(GCThreadFunction function, void* param, GCThreadAffinity* affinity);
                                                                           ^

[parjong]

\CC @seanshpark

[janvorli]

LGTM, thank you!

[janvorli]

cc: @Maoni0, @swgillespie