invoke.texi: Clarify -march=lujiazui

invoke.texi: Clarify -march=lujiazui

I was recently searching which exact CPUs are affected by the PR114576
wrong-code issue and went from the PTA_* bitmasks in GCC, so arrived
at the goldmont, goldmont-plus, tremont and lujiazui CPUs (as -march=
cases which do enable -maes and don't enable -mavx).
But when double-checking that against the invoke.texi documentation,
that was true for the first 3, but lujiazui said it supported AVX.
I was really confused by that, until I found the
https://gcc.gnu.org/pipermail/gcc-patches/2022-October/604407.html
explanation.  So, seems the CPUs do have AVX and F16C but -march=lujiazui
doesn't enable those and even activelly attempts to filter those out from
the announced CPUID features, in glibc as well as e.g. in libgcc.

Thus, I think we should document what actually happens, otherwise
users could assume that
gcc -march=lujiazui predefines __AVX__ and __F16C__, which it doesn't.

2024-06-04  Jakub Jelinek  <jakub@redhat.com>

	* doc/invoke.texi (lujiazui): Clarify that while the CPUs do support
	AVX and F16C, -march=lujiazui actually doesn't enable those.