config: add aarch64 clang for Ubuntu 18.04

config: add aarch64 clang for Ubuntu 18.04

Create distribution specific meson cross-file
arm64_armv8_linux_clang_ubuntu1804. The file is distribution specific
because it contains paths to headers and libs specific to the
distribution. The clang/LLVM toolchain does not provide its own c stdlib
so the paths must be supplied in some manner.

In the current version of meson, v0.47.1, the only place
where the paths can be specified is the cross-file. Other possibilities
do not work:
* setting CFLAGS, LDFLAGS only sets these for non-cross builds.
* setting -Dc_args, -Dc_link_args on the command line also only sets
these for non-cross builds. Support for specifying these for
cross builds was added in v0.51.0 [0].
* the cross-file can't be split into generic clang cross config and
distribution specific config. Support added in v0.52.0 [1].

[0] https://mesonbuild.com/Builtin-options.html#specifying-options-per-machine
[1] https://mesonbuild.com/Machine-files.html#loading-multiple-machine-files

Signed-off-by: Juraj Linkeš <juraj.linkes@pantheon.tech>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>