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
Change-Id: I9866c3579a7a94de27bfbe80ad7a822c3183c7fb