i#1551 port to ARM: add Linux ARM syscall numbers

Move our own syscall.h header to syscall_linux_x86.h, add
syscall_linux_arm.h, and add top-level syscall.h that selects which of the
two to use.

Adds SYS_ versions of the 5 ARM-only syscalls, which are missing from the
/usr/include headers.

Adds ifdefs around the syscalls that DR processes that are not present on
ARM.

Replaces DR's use of SYS_time as it is not supported on ARM.

Review-URL: https://codereview.appspot.com/188140043