powerpc: trap_is_syscall() helper to hide syscall trap number

A new system call interrupt will be added with a new trap number.
Hide the explicit 0xc00 test behind an accessor to reduce churn
in callers.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Make it a static inline]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200507121332.2233629-3-mpe@ellerman.id.au