In a bootstrapping build with LLVM_ENABLE_PROJECTS=lld, the lld
executable will be found in LLVM's bin/ directory. But `check-offload`
will currently ignore it because the JIT plugin will look for `lld` in
`$PATH`. Similarly, the sanitizer tests require llvm-symbolizer during
execution to FileCheck the expected stack trace.
Add the llvm tools directory to `$PATH` so tests can use lld and
llvm-symbolizer in it. It should be prefered over a system-installed
lld/llvm-symbolizer.
Fixes the JIT and sanitizer tests of
[openmp-offload-amdgpu-clang-flang](https://lab.llvm.org/staging/#/builders/105).
0e1cc6138e