[klibc] syscalls: Remove support for socketcall()

All architectures that implement socketcall() now also have separate
system calls:

- arm: added in 2.6.12
- i386: added in 4.3
- m68k: added in 4.3
- mips, mips64: added in 2.1.44
- parisc: always had both
- ppc, ppc64: added in 2.6.37
- s390, s390x: added in 4.3
- sh: added in 2.6.37
- sparc, sparc64: added in 4.4 and 4.20

We can therefore unconditionally use the separate system calls, except
for accept() which may need to be a wrapper for accept4().

Remove support for socketcall(), and move the socket system call
definitions into SYSCALLS.def.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>