main now requires each exported function to be declared once, in its
library's public header. The guest main stack test declared five libkernel
functions itself, one of them with a different signature from Api.hpp.
Include Api.hpp instead, and declare EnterGuestMainStack_nid_no_patch once in
Pthread.hpp, taking the entry as an APS5_VABI function pointer as the relinker
test host's stub does.