obtain Windows syscall numbers offline

With the frequent major win10 updates, adding new tables to
core/win32/syscallx.h and core/win32/syscall.c is getting tedious and taking up
space. Should we stop adding the win10 updates here and give up on our table of
numbers, relying on reading the wrappers (#1598 changed DR to work purely on
wrapper-obtained numbers)? We'd lose robustness vs hooks, and clients like
Dr. Memory who have to distinguish win10 versions would have to do their own
versioning. I guess we could still have DR_WINDOWS_VERSION_xx and not have
corresponding tables here. Or we could go the planned Dr. Memory route
(https://github.com/DynamoRIO/drmemory/issues/1848) and store these numbers in a
separate file that is updated via a separate standalone utility run once by the
user.

Xref #1854