[llvm][lit] Handle case when there is no llvm default target triple (#76934)

This can happen when you do not choose a specific target triple, and do
not enable the host architecture when building (if you do enable it, it
would become the default target). Such as only enabling RISC-V, when
building on an AArch64 machine.

Originally reported
https://discourse.llvm.org/t/llvm-test-error-could-not-turn-into-itanium-abi-triple/76013.

When attempting to run a single test via lit you get: `Could not turn ''
into Itanium ABI triple`

Setting a default triple with `LLVM_DEFAULT_TARGET_TRIPLE` works around
the issue.

This change copies the existing host triple check to target triple, and
adds a note to highlight the potential issue. As `check-clang` on my
AArch64 machine failed 32% of tests in this configuration.

Which is to be expected and is ok if you only want to run specific
tests, but for anyone unintentionally building this way the note is a
clue to the cause.