syscall(3)'s return type should be long.

This doesn't require us to change any of the syscall implementations
because (a) the LP32 ones have sizeof(int) == sizeof(long) anyway,
which is how we never noticed this bug before and (b) the LP64 ones
all use a 64-bit register for the result (and for the syscall number
too).

Bug: https://code.google.com/p/android/issues/detail?id=73952
Bug: 16568314

(cherry picked from commit 21972b61)

Change-Id: Ifbc424be29e5650ec72a24df25dd35f24fdd5b3c