package/llvm: new package

package/llvm: new package

This patch installs LLVM tools and libraries for the host and
libLLVM.so for the target.

In order to cross-compile LLVM for the target, LLVM
must be installed on the host, or at least llvm-tblgen.
This is necessary as the path to host's llvm-tblgen must
be specified when cross-compiling using the LLVM_TABLEGEN option.
Also, a version of llvm-config that can run on the host will
be required by packages that link with LLVM libraries, so we
need to generate it and install it in STAGING_DIR/usr/bin.

It is important to remark why we need llvm-config(host variant)
installed in STAGING dir. This tool is necessary to build
applications that use LLVM, as it prints the compiler flags,
linker flags and object libraries needed to link against LLVM libs.

More info: https://bugs.chromium.org/p/chromium/issues/detail?id=219369

The original idea was to compile only llvm-tblgen and llvm-config
for the host, as they are the only necessary components. However,
llvm-config tool does not work as expected if it is not linked with
libLLVM.so, so we must also enable LLVM_LINK_LLVM_DYLIB, what builds
LLVM as a single shared library and links LLVM tools with it.

More info: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=224847
in comment #11.

If we don't build full LLVM for the host, it would be necessary to
patch configure.ac from mesa3d if we want dynamic linking, because it
uses llvm-config (host variant installed in STAGING_DIR) to get the
necessary LLVM libraries to link with, which has the following problems:

 - llvm-config --shared mode outputs static (even if LLVM is built as
   one shared library) which leads to link issues with libgallium.

 - llvm-config --libs outputs all LLVM tiny libs: -lLLVMLTO,
   -lLLVMPasses,etc instead of the single shared library containing
    all LLVM components (-lLLVM-5.0)

Mesa tries to execute: llvm-config --link-shared --libs, but this outputs
llvm-config: error: libLLVM-5.0.so is missing.

Given that these problems may arise with other packages that use LLVM,
it is preferable to do a full build for the host. Also, having a
complete installation of LLVM on the host will also facilitate the
integration of Clang front-end, which is going to be added in a future
patch.

As option LLVM_BUILD_LLVM_DYLIB is enabled for the llvm target variant,
a single shared library containing all LLVM components is built.
This option is not compatible with BUILD_SHARED_LIBS, which generates
one .so per library and is only recommended for use by LLVM developers.

Tools and utils are not built for the target. The patch aims to provide
LLVM support for other packages.

The main options needed to cross-compile LLVM are the following ones:

LLVM_TABLEGEN
CMAKE_CROSSCOMPILING
LLVM_DEFAULT_TARGET_TRIPLE
LLVM_HOST_TRIPLE
LLVM_TARGET_ARCH
LLVM_TARGETS_TO_BUILD

Signed-off-by: Valentin Korenblit <valentin.korenblit@smile.fr>
[Thomas:
 - add dependency on thread and C++ and update the Config.in comment
   accordingly.
 - make the Config.in comment depend on BR2_PACKAGE_LLVM_ARCH_SUPPORTS
   so that it isn't disabled on architectures where LLVM is anyway not
   supported.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>