Framework: Switch LINUX_DEFAULT to rl9 for all arches except i386

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Summary

The current Linux base we use in Ports comes from Cent OS 7, which is
not only ancient by itself, but also have gone EoL.

This change switches the default to a more recent Rocky Linux 9 base. There
are several points that should be acknowledged with this change:

• LINUX_DEFAULT for ARCH=i386 stays to be "c7". This is because Rocky Linux has no separate i386 target, but ships 32-bit binaries from 64-bit package repos. It is probably possible to properly support i386 with rl9, but no one did the job.
• I was told that 64-bit binaries that are shipped in rl9 require SSE 4.2, which means that owners of old amd64 hardware (pre Nehalem?) would need to switch the default back.

Test Plan

No exp-run is needed, because 99% of Linux ports are just repackaged
rather than compiled. So I simply gone over a list of USES=linux ports and
verified that they still run with rl9. Those who broked are now marked USES=linux:c7.

The ports I wasn't able to test are:

Require CDROM:
games/linux-ssamtfe
games/linux-ssamtse
games/linux-unrealgold
games/linux-ut

Unfetchable:
java/linux-oracle-jdk18
java/linux-oracle-jre18
print/epson-inkjet-printer-201207w
print/epson-inkjet-printer-201401w
print/epson-inkjet-printer-201601w
print/epson-inkjet-printer-escpr2

Simply don't work:
games/linux-doom3
games/linux-nwnclient

Since games/linux-doom3-demo works I assume games/linux-doom3 should be fine
too. Not sure what to do about games/linux-nwnclient.

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games/nwnclient will be fixed after the switch according to https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=252747