dependencies/LLVM: rework shared link detection to be more robust

Instead of trying to hardcode which versions of which OSes are
misconfiguring llvm-config, lets try to use a generic mechanism that
catches most of the broken cases.

If a dynamic lib is built but the tools are linked staticly
(LLVM_BUILD_LLVM_DYLIB=true and LLVM_LINK_LLVM_DYLIB=false) and you
don't enabled shared libs (LLVM_BUILD_SHARED_LIBS=false, which should be
false), then llvm-config will mistakenly return a each static library
with .(so|dll|dylib) appended, instead of -lLLVM-x.y.z. if
LLVM_LINK_LLVM_DYlIB is true this works fine. There's been a bug opened
in upstream since 2014 about this with no responses[1].

So, this patches looks at --shared-mode (which tells us whether
llvm-config was linked staticly or dynamicly), then if the shared-mode
is static it will check and see what it gets. If it gets valid results
then it will go with those, if it doesn't then it will try to build a
set of valid link arguments to link with libLLVM.(so|dll|dylib).

[1] https://bugs.llvm.org/show_bug.cgi?id=19937

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