sci-physics/root: switch back to builtin LLVM

sci-physics/root: switch back to builtin LLVM

Using external LLVM never really worked and can cause crashes
if ROOT is used with anything else that dynamically links against
LLVM, such as media-libs/mesa[llvm]. The bundled version of clang
is also API incompatible with vanilla clang and doesn't work as a
compiler with the applied patches that are required to make the
cling C/C++ interpreter work.

More information can be found in the links below:

Bug: https://bugs.gentoo.org/564302
Bug: https://bugs.gentoo.org/687212
Bug: https://sft.its.cern.ch/jira/browse/ROOT-9965
Bug: https://sft.its.cern.ch/jira/browse/ROOT-10287
Bug: https://cern.service-now.com/service-portal/view-incident.do?n=INC1242141 (requires authentication)
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=93103
Bug: https://bugzilla.redhat.com/show_bug.cgi?id=1441957 (restricted)
Package-Manager: Portage-2.3.71, Repoman-2.3.17
Signed-off-by: Guilherme Amadio <amadio@gentoo.org>