setup LLVM=1 tests for 5.4 forward

[nickdesaulniers]

In accordance with https://android-review.googlesource.com/c/platform/prebuilts/clang/host/linux-x86/+/1405387 and in time for plumbers in order to discuss with CI system maintainers, I'd like to get LLVM=1 builds setup for as many arches as possible with 5.4+. This will require backporting

fcf1b6a35c16ac500fa908a4022238e5d666eabf
0f44fbc162b737ff6251ae248184390ae2279fee
63b903dfebdea92aa92ad337d8451a6fbfeabf9d
734f3719d3438f9cc181d674c33ca9762e9148a1
eefb8c124fd969e9a174ff2bedff86aa305a7438
94f7345b712405b79647a6a4bf8ccbd0d78fa69d
aa824e0c962b532d5073cbb41b2efcd6f5e72bae
7e20e47c70f810d678d02941fa3c671209c4ca97
a0d1c951ef08ed24f35129267e3595d86f57f5d3
to 5.4.y.

[nickdesaulniers]

https://lore.kernel.org/stable/CAKwvOd=Ko_UHWF-bYotqjPVw=chW_KMUFuBp_o8uOg0wOyHyWA@mail.gmail.com/T/#u

[nickdesaulniers]

https://lore.kernel.org/lkml/20200820220955.3325555-1-ndesaulniers@google.com/T/#u