Use IMP_LIB_EXT when checking for LLVM shared libraries

Use IMP_LIB_EXT when checking for LLVM shared libraries

When checking for LLVM shared libraries, use IMP_LIB_EXT for the extension for
shared libraries appropriate to the target, rather than hardcoding '.so'

Also add some comments to explain why we have this circus of pain.

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Tom Stellard <thomas.stellard@amd.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>