Add unwind information for r7 in arm syscalls.

It turns out that clang can emit code where the sp is saved in the r7
register on arm. Unfortunately, a lot of our syscalls overwrite that
value while the syscall is executing, so unwinding through that syscall
fails.

Update the syscall generation code to add unwinding information for
these uses.

Bug: 28411713
Change-Id: Id51a5d43c058631bb50834527f93fa299bbcf759