R600/SI: Use mad for fsub + fmul

We can use a negate source modifier to match
this for fsub.
Make fabs safe to speculatively execute
lldb - Register Context Linux ARM64

Yet another step toward ARM64 support. With this commit, lldb-gdbserver started on ARM64 target can be accessed by lldb running on desktop PC and it can process simple commands (like 'continue'). Still ARM64 support lacks NativeRegisterContextLinux_arm64.* code which waits to be implemented.
Based on similar files for Linux x86_64 and Darwin ARM64. Due to common code extraction from Darwin related files, lldb should be tested for any unexpected regression on Darwin ARM64 machines too.

See the following for more details:
http://reviews.llvm.org/D4580
http://lists.cs.uiuc.edu/pipermail/lldb-commits/Week-of-Mon-20140825/012670.html

Change by Paul Osmialowski.