[clangd] Strip mutliple arch options

This patch strips all the arch options in case of multiple ones. As it
results in multiple compiler jobs, which clangd cannot handle.

It doesn't pick any over the others as it is unclear which one the user wants
and defaulting to host architecture seems less surprising. Users also have the
ability to explicitly specify the architecture they want via clangd config
files.

Fixes https://github.com/clangd/clangd/issues/827.

Differential Revision: https://reviews.llvm.org/D107634