alpha: add generic get{eg,eu,g,p,u,pp}id() syscalls

Alpha has traditionally followed the OSF1 calling conventions
here, with its getxpid, getxuid, getxgid system calls returning
two different values in separate registers.

Following what glibc has done here, we can define getpid,
getuid and getgid to be aliases for getxpid, getxuid and getxgid
respectively, and add new system call numbers for getppid, geteuid
and getegid.

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