The deduced "-march=" option always started with aarch64, which is not a
valid value. There was also no way to distinguish between armv8-r and
armv8-a. After this commit, the deduced "-march=" option will start with
greatest available "armv*-a" value or "armv8-r".
When the FPU was selected with "+(no)fp(.dp)" extensions in "-march" or
"-mcpu" options, the FPU used for multilib selection was still the
default one for given architecture or CPU.
This option causes the flags used for selecting multilibs to be printed.
This is an experimental feature that is documented in detail in D143587.
Differential Revision: https://reviews.llvm.org/D142933