[HIP] Supports env var HIP_PATH

Currently HIP toolchain recognize env var ROCM_PATH and option --rocm-path
but only recognize --hip-path.

Some package management tools e.g. Spack relies on env var HIP_PATH to
be able to load different version of HIP dynamically.
(https://github.com/spack/spack/blob/develop/var/spack/repos/builtin/packages/hip/package.py#L446)
Therefore add support of env var HIP_PATH.

Reviewed by: Artem Belevich, Fangrui Song

Differential Revision: https://reviews.llvm.org/D145391