All ebuilds that could satisfy "app-admin/eselect-compiler" have been masked.
while trying to update system

Fix/Answer

run
emerge -C eselect-compiler && emerge =gcc-config-1*

Description:

Because of long outstanding bugs and problems the eselect-compiler tools have been masked in the portage tree.
glibc requires either gcc-config or eselect-compiler.
If you had eselect-compiler installed, glibc complains.

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