asm-generic: unistd.h: allow generating syscall.tbl from header

The asm-generic/unistd.h header is almost in the state where it can be
converted into a syscall.tbl file using just the C preprecessor. Add
the last missing bits by always defining the syscall numbers for all
ABIs if __GEN_SYSCALL_TBL is defined as a macro, and add an
alternative __SC/__SCC pair of macros that evaluates to the
normal format.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>