The system variable CHOST tells the c compiler for what it has to compile. Once set, modifying require a sequence of steps until the complete system gets aware of the change.

Recompiling everything using emerge --ask --emptytree @world might be necessary. See: https://wiki.gentoo.org/wiki/Changing_the_CHOST_variable

On newer profile CHOST does not have to be set in /etc/portage/make.conf since the profiles have a default value.

CHOST contains different fields see: https://wiki.gentoo.org/wiki/CHOST