https://sourceware.org/bugzilla/show_bug.cgi?id=26703 deprecated the
previous GNU_PROPERTY_X86_ISA_1_{CMOV,SSE,*} values (renamed to COMPAT)
and added new values.
Since the legacy values are not used by compilers, having dumping support in
llvm-readobj is unnecessary. So just drop the legacy feature.
The new values are used by GCC 11
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97250) -march=x86-64-v[234] to
indicate the micro-architecture ISA levels.