Remove hardcoded registers from Hexagon ABI

Summary: The Hexagon ABI plugin uses hardcoded registers when setting up function calls. This is OK for the Hexagon simulator, but the register numbers are different on the gdbserver running on hardware. Change the hardcoded registers to LLDB generic registers.

Reviewers: clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D15457

llvm-svn: 255374