gcc-4.8.3 rebuilds were triggered by a new 'sanitize' USE flag. However, the ebuild no longer compiles for MinGW targets (it did in the past). Whether USE=sanitize is set or not seems to be irrelevant.
Created attachment 387016 [details] emerge --info
Created attachment 387018 [details] build.log.xz
http://sources.gentoo.org/eclass/toolchain.eclass?r1=1.639&r2=1.640