OSL_THIS_FUNC: use logic from boot/current_function.hpp
OSL_THIS_FUNC: use logic from boot/current_function.hpp

Don't use it directly, as OSL_THIS_FUNC is used in C-only sources as
well. Also, remove OSL_THIS_FUNC from OSL_LOG_PREFIX as concatenation
does not work non-macros like __PRETTY_FUNCTION__. (The code actually
worked there only because it was defined to be " ", so we are not
removing any valuable information.)

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core