Provide inline definitions of _Unwind_GetIP etc. for ARM in unwind.h

These symbols were showing up as undefined when trying to link programs on
Android. We should match libgcc's behaviour and provide inline definitions
of these on ARM.

It seems unwind.h on ARM/Darwin doesn't provide inline definitions, so we
just declare them for that platform.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191406 91177308-0d34-0410-b5e6-96231b3b80d8