[libclc][amdgpu] Implement native_exp2 via AMD builtin (#133696)

This came up during a discussion on #129679, which has been split out as
a preparatory commit.

An example of the AMDGPU codegen is:

    define <2 x float> @_Z10native_expDv2_f(<2 x float> %val) {
      %mul = fmul afn <2 x float> %val, splat (float 0x3FF7154760000000)
      %0 = extractelement <2 x float> %mul, i64 0
      %1 = tail call float @llvm.amdgcn.exp2.f32(float %0)
      %vecinit.i = insertelement <2 x float> poison, float %1, i64 0
      %2 = extractelement <2 x float> %mul, i64 1
      %3 = tail call float @llvm.amdgcn.exp2.f32(float %2)
%vecinit2.i = insertelement <2 x float> %vecinit.i, float %3, i64 1
      ret <2 x float> %vecinit2.i
    }

    define <2 x float> @_Z11native_exp2Dv2_f(<2 x float> %x) {
      %0 = extractelement <2 x float> %x, i64 0
      %1 = tail call float @llvm.amdgcn.exp2.f32(float %0)
      %vecinit = insertelement <2 x float> poison, float %1, i64 0
      %2 = extractelement <2 x float> %x, i64 1
      %3 = tail call float @llvm.amdgcn.exp2.f32(float %2)
      %vecinit2 = insertelement <2 x float> %vecinit, float %3, i64 1
      ret <2 x float> %vecinit2
    }

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