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>

config/arm/arm64_armv8_linux_clang_ubuntu1804

@@ -0,0 +1,20 @@
+[binaries]
+c = 'clang'
+cpp = 'clang++'
+ar = 'llvm-ar'
+strip = 'llvm-strip'
+llvm-config = 'llvm-config'
+pcap-config = 'llvm-config'
+pkgconfig = 'aarch64-linux-gnu-pkg-config'
+
+[host_machine]
+system = 'linux'
+cpu_family = 'aarch64'
+cpu = 'armv8-a'
+endian = 'little'
+
+[properties]
+implementor_id = 'generic'
+implementor_pn = 'default'
+c_args = ['-target', 'aarch64-linux-gnu', '--sysroot', '/usr/aarch64-linux-gnu']
+c_link_args = ['-target', 'aarch64-linux-gnu', '-fuse-ld=lld', '--gcc-toolchain=/usr']