gdb/testsuite/lib/my-syscalls.S: Refactor new SYSCALL macro

gdb/testsuite/lib/my-syscalls.S: Refactor new SYSCALL macro

Refactor the syscall assembly code in gdb/testsuite/lib/my-syscalls.S
behind a SYSCALL macro so that it's easy to add new syscalls without
duplicating code.

Note that the way the macro is implemented, it only works correctly
for syscalls with up to 3 arguments, and, if the syscall doesn't
return (the macro doesn't bother to save/restore callee-saved
registers).

The following patch will want to use the macro to define a wrapper for
the "exit" syscall, so the limitations continue to be sufficient.

Change-Id: I8acf1463b11a084d6b4579aaffb49b5d0dea3bba
Reviewed-By: Andrew Burgess <aburgess@redhat.com>