[test] Run llvm/test/**/*.yaml & don't run llvm/test/**/*.cxx (not exist)

[test] Run llvm/test/**/*.yaml & don't run llvm/test/**/*.cxx (not exist)

This patch extends D58439 (`llvm/test/{yaml2obj,obj2yaml}/**/*.yaml`) and runs all
`llvm/test/**/*.yaml`

Many directories have configured `.yaml` (see the deleted lit.local.cfg
files). Yet still some don't configure .yaml and have caused stale tests:

* 8c5825befb7bbb2e76f7eccedc6d3bf26e9b2a6a test/llvm-readobj
* bdc3134e237737dd46b51cd1ecd41ecbbe9f921a test/ExecutionEngine

Just hoist .yaml to `llvm/test/lit.cfg.py`. Also delete .cxx which is
not used.  The number of tests running on my machine increases from 38304 to 38309.
The list of new tests:

```
ExecutionEngine/RuntimeDyld/X86/ELF_x86-64_none.yaml
Object/archive-error-tmp.txt
tools/llvm-ar/coff-weak.yaml
tools/llvm-readobj/ELF/verneed-flags.yaml
tools/obj2yaml/COFF/bss.s
```

Reviewed By: grimar, jhenderson, rupprecht

Differential Revision: https://reviews.llvm.org/D83350