clang: tell cross-builds of clang how to build natively

clang: tell cross-builds of clang how to build natively

If we pass CROSS_TOOLCHAIN_FLAGS_NATIVE to all the builds, instead of
just nativesdk, then clang can correctly build its own native tools as
needed.

This means we can stop manually installing clang-tidy-confusable-chars-gen
as clang-tidy will build it.  We (currently) still build the tblgen
binaries once as they're used in several recipes so there's likely value
in building those once.

Also consolidate the LLVM_TABLEGEN variables as they're not specific to
a particular class.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>