llvm: set LLVM_HOST_TRIPLE for cross-compilation

llvm: set LLVM_HOST_TRIPLE for cross-compilation

The correct way to configure cmake for cross-compilation includes
setting the LLVM_HOST_TRIPLE as well.

Documentation Ref:
https://github.com/llvm/llvm-project/blob/llvmorg-19.1.6/llvm/docs/HowToCrossCompileLLVM.rst#configuring-cmake

This fixes bug when LLVM is cross-compiled for AAarch64 but it
was getting built with the triple of native build system instead of
the TARGET_SYS (aarch64-oe-linux) in my case.

`No available targets are compatible with triple "x86_64-unknown-linux-gnu"`

Signed-off-by: Haseeb Ashraf <haseeb.ashraf@siemens.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>