Add four new functions and one new enum to the C API:

LLVMGetThreadLocalMode - exposes GlobalVariable::getThreadLocalMode
LLVMSetThreadLocalMode - exposes GlobalVariable::setThreadLocalMode
LLVMIsExternallyInitialized - exposes GlobalVariable::isExternallyInitialized
LLVMSetExternallyInitialized - exposes GlobalVariable::setExternallyInitialized
LLVMThreadLocalMode - maps to GlobalVariable::ThreadLocalMode

Patch by Moritz Maxeiner!