[HIP] use GetProgramPath for executable discovery
This change replaces the manual building of executable paths
using llvm::sys::path::append with GetProgramPath.
This enables adding other paths in case executables reside
in different directories and makes the code easier to read.
Differential Revision: https://reviews.llvm.org/D72903
(cherry picked from commit 24d7a0935bea390fc444f6ac8cf02447fe8d6917)