x86: always set ISA_1_BASELINE property for 64-bit objects

The baseline was, afaik, specifically chosen to align with the baseline
ISA of x86-64. It therefore makes no sense to emit that property only
conditionally; if anything it confuses tools analyzing the difference
between generated object files, which may result from just
added / changed / removed (entirely ISA-independent) code, without any
change to the enabled extensions. Compilers, after all, are free to use
these baseline "extensions" when generating 64-bit code.

While changing the one testcase that needs adjustment, also correct its
misleading name (to be in sync with the filename).