Add two new calling conventions for runtime calls

This patch adds two new target-independent calling conventions for runtime
calls - PreserveMost and PreserveAll.
The target-specific implementation for X86-64 is defined as following:
  - Arguments are passed as for the default C calling convention
  - The same applies for the return value(s)
  - PreserveMost preserves all GPRs - except R11
  - PreserveAll preserves all GPRs and all XMMs/YMMs - except R11

Reviewed by Lang and Philip

llvm-svn: 199508