mmap() can't be called via (__)syscall

[arichardson]

This is used e.g. by the sanitizer runtimes, but since unistd.h declares syscall/__syscall with an integer return value it's not possible to actually call mmap() successfully:

cheribsd/include/unistd.h

Lines 582 to 583 in 698d163

	int syscall(int, ...);
	off_t __syscall(quad_t, ...);

Should these be changed to intptr_t for purecap?

[brooksdavis]

I suspect we should leave syscall alone (it can't be used for mmap except by accident), but likely __syscall should be int64ptr_t.

[jrtc27]

Using it for mmap has two problems:

1. off_t is 64-bit, so on a 32-bit system you need to use __syscall so the arguments are aligned correctly
2. Pointers get truncated on a 64-bit system even absent CHERI

There's also shmat which suffers from 2 but not 1.