Fix up the "generic" unistd.h ABI to be more useful.

Reserve 16 "architecture-specific" syscall numbers starting at 244.

Allow use of the sys_sync_file_range2() API with the generic unistd.h
by specifying __ARCH_WANT_SYNC_FILE_RANGE2 before including it.

Allow using the generic unistd.h to create the "compat" syscall table
by specifying __SYSCALL_COMPAT before including it.

Use sys_fadvise64_64 for __NR3264_fadvise64 in both 32- and 64-bit mode.

Request the appropriate __ARCH_WANT_COMPAT_SYS_xxx values when
some deprecated syscall modes are selected.

As part of this change to fix up the syscalls, also provide a couple
of missing signal-related syscall prototypes in <linux/syscalls.h>.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>