[Bug target/97585] Improve documentation for -march=x86-64 to say MMX, SSE, SSE2 are implied

[sjames at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97585

Sam James <sjames at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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   Target Milestone|---                         |15.0