[Clang] Search for 'offload-arch' only next to the clang driver (#150965)
Summary:
Previously, querying for the offload architecture tool would invoke the
user's PATH, which is bad when potentially using the driver from a
direct path. This patch change this to *only* consider the
`offload-arch` that's supposed to live next to the driver executable.
Now we will no longer pick up a potentially conflicting version of this
tool and it should always be found (Since it's a clang tool that's
installazed alongside the driver)