Add definition for M_PI_F v3

v2:
  - Use a hexadecimal constant.

v3:
  - Use a hexadecimal constant in floating-point notation.

git-svn-id: https://llvm.org/svn/llvm-project/libclc/trunk@204666 91177308-0d34-0410-b5e6-96231b3b80d8