update build infrastructure for llvm-16.0.6

Update clang build infrastructure for LLVM 11.1.0.

Heavy lifting by mortimer@, thank you so much!

Update build infrastructure for LLVM 10.0.0.

ok hackroom@
tested by plenty

add Makefile for llvm-config

A build time dependency on python is avoided by generating the arch
specific list of library components in advance. A 'reconf' target
is included to regenerate them. A

add Makefile for llvm-config

A build time dependency on python is avoided by generating the arch
specific list of library components in advance. A 'reconf' target
is included to regenerate them. Approach discussed with patrick@

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