Add a new unit test, testpatchentrypoints_gldispatch. It calls into
libGLdispatch directly instead of going through GLX or EGL, and it tests
patching and unpatching between multiple vendor libraries.
Rewrote the x86-64 assembly for the entrypoint patching tests. It now uses
absolute memory addresses instead of RIP-relative, so it can work even if the
variable that it's supposed to update isn't within +-2GB of the function.
