meson: Make shared-llvm into a tri-state boolean

meson: Make shared-llvm into a tri-state boolean

Choosing LLVM's link mode is legitimate on UNIX systems, but only static
actually really works under Windows.

Give shared-llvm a default 'auto' mode which will pick the previous
default of true (shared) on UNIX systems, but newly defaulting to false
(static) on Windows.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Suggested-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4555>