make arm-reserve-r9 available for all ARM

r9 is defined as a platform-specific register in the ARM EABI.
It can be reserved for a special purpose or be used as a general
purpose register. Add support for reserving r9 for all ARM, while
leaving the IOS usage unchanged.

Patch by Jeroen Hofstee.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188485 91177308-0d34-0410-b5e6-96231b3b80d8