I've prepared external repository with GCC 8 built from trunk (as there is no stable release yet) that also contains glibc 2.27 and binutils 2.30.
[gcc8] Server = http://pkgbuild.com/~bpiotrowski/gcc8/
From less obvious packaging changes, glibc no longer ships with obsolete rpc and nsl, which means that you should switch your packages to libtirpc; also if your nsswitch.conf still contains "compat" instead of "files", better fix it before reboot. If for some reason you still use nsl, the repository also ships libnsl and libnsl_nis packages.
As usually, simple things build, and colossi like LibreOffice don't, but I haven't had time yet to check why. If you found some issue, the best would be to reply either here or arch-general.