I've committed this patch to set the minimum required LLVM version, for 
the assembler and linker, to 13.0.1. An upgrade from LLVM 9 is a 
prerequisite for the gfx90a support, and 13.0.1 is now the oldest 
version not known to have compatibility issues.

The patch removes all the obsolete feature detection tests from 
configure and adds a new version test. Likewise the version dependencies 
in the backend are removed.

Andrew