Upgrade gcc version/tweak package installs in Gentoo installation (#8757)

GCC 4.9.4 is no longer available on Gentoo (or Sabayon), which causes
problems when attempting to install on either of these platforms. Since
QMK is not particularly sensitive to its GCC version, modify the version
restriction to <9 so newer versions of GCC may be installed. Since the
toolchain for arm-none-eabi isn't currently installed as part of setup,
add that as well.

Additionally, drop the Python installation as part of the Gentoo
installation process. Python is a core system package on Gentoo and can
therefore be assumed to be present; in addition, the slot restriction of
3.5 which was present is also no longer available in Gentoo.

Finally, separate the gcc rebuild invocation of `emerge` from the new
packages that may need to be installed, and apply the `--noreplace` flag
to new packages so that they are not rebuilt if already present.

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