sys-devel/llvm: Fix bogus flags and paths in llvm-config, #565358

sys-devel/llvm: Fix bogus flags and paths in llvm-config, #565358

Fix llvm-config to avoid bogus results. In particular:

1. Limit --cflags and --cxxflags to package-specific flags. Do not
output the whole flag-string used during the build. This fixes libclc
build issues when LLVM build flags were not tolerated by clang.

2. Fix library names and paths to use shared library suffix rather than
static library suffix, especially that we do not install static
libraries.

3. Wipe out --system-libs since they should not be required for dynamic
linking.

4. Ban --obj-root and --src-root when running outside source tree, since
we are not installing any sources and therefore their results would
always be bogus.

Based on patch provided by Steven Newbury.

Fixes: https://bugs.gentoo.org/565358