The goal of this patch is to get rid of some of the literal consts
used in process record -- specifically the ones used for syscalls.

It's a big patch, but mostly mechanical.  This explanation is
meant to make it easier to review.

1) The amd64 linux syscall consts are extracted from amd64-linux-tdep.c
into an enum typedef in amd64-linux-tdep.h

2) The canonical gdb syscall consts are extracted from linux-record.c
and amd64-linux-tdep.c into an enum typedef in linux-record.h.

3) The switch statement in amd64_linux_syscall_record that uses
all of these consts is abstracted into a new function, and the
constants are replaced.

4) The constants are replaced in linux-record.c: record_linux_system_call.

5) Finally one literal const (499) in i386_linux_tdep.c:
i386_linux_intx80_sysenter_record is also replaced.
I deliberately didn't change anything else in that function,
because there's already another patch in review for it.

Reverse testsuites show no regressions on i386 or amd64.