Rename bit_RDRAND to bit_RDRND to match GCC's version of this header.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218823 91177308-0d34-0410-b5e6-96231b3b80d8
Add the various signature macros.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218824 91177308-0d34-0410-b5e6-96231b3b80d8