libcxx: Pass LLVM_RUNTIME_TARGETS

LLVM_RUNTIME_TARGETS will define the location of triple for target
correctly. This is an effort to fix the case where cross compiler will
lurk into host libc++ header if libcxx-native is staged for a given
recipe e.g. graphviz, it works ok if --sysroot is specified because
thats how it tries to find if its a cross sysroot or not but without
that it has mind of its own.

e.g. try
x86_64-yoe-linux-musl-clang++ -stdlib=libc++ ~/tests/iosfd.cpp -v -c

...
clang -cc1 version 18.1.0 based upon LLVM 18.1.0 default target x86_64-unknown-linux-gnu
ignoring nonexistent directory "/usr/lib64/gcc/x86_64-pc-linux-gnu/13.2.1/../../../../x86_64-pc-linux-gnu/include"
ignoring nonexistent directory "/include"
 /mnt/b/yoe/master/build/tmp/work/core2-64-yoe-linux-musl/graphviz/8.1.0/recipe-sysroot-native/usr/bin/../include/c++/v1
 /usr/local/include
 /usr/include
 /mnt/b/yoe/master/build/tmp/work/core2-64-yoe-linux-musl/graphviz/8.1.0/recipe-sysroot-native/usr/lib/clang/18/include
End of search list.
...

See where it is looking for c++ headers.

Its not an issue for OE right now, since we always specify --sysroot in
CC but still would be good to handle it amicably if possible

Signed-off-by: Khem Raj <raj.khem@gmail.com>

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