[NVPTX] Add ex2.approx.f16/f16x2 support

NOTE: this is a follow-up commit with the missing clang-side changes.

This patch adds builtins and intrinsics for the f16 and f16x2 variants of the ex2
instruction.

These two variants were added in PTX7.0, and are supported by sm_75 and above.

Note that this isn't wired with the exp2 llvm intrinsic because the ex2
instruction is only available in its approx variant.

Running ptxas on the assembly generated by the test f16-ex2.ll works as
expected.

Differential Revision: https://reviews.llvm.org/D119157