Create a distinct temp directory for each run.

Modify make__NR_name so that only __ARM_NR_ is exempted from the
__NR_ being prepended. This avoids a case where using a name starting
with __ but is not a valid syscall name in SYSCALLS.TXT does not generate
code that will compile but references the function itself and causes
link errors.

Fix all of the directory references from dir_part1 + dir_part2 to
use os.path.join() instead.

Change-Id: Ib9527eba6f25f26a30c5cb0ad431f3f88a7683cf