loongarch: convert to generic syscall table

The uapi/asm/unistd_64.h and asm/syscall_table_64.h headers can now be
generated from scripts/syscall.tbl, which makes this consistent with
the other architectures that have their own syscall.tbl.

Unlike the other architectures using the asm-generic header, loongarch
uses none of the deprecated system calls at the moment.

Both the user visible side of asm/unistd.h and the internal syscall
table in the kernel should have the same effective contents after this.

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