glibc 2.26 supports compiling with versions of gcc 4.9 and later, however once it is installed, gcc itself can not be recompiled on all but the most recent versions of gcc. This means that the system can not longer rebuild itself if an older unpatched gcc is installed. The toolchain should always be able to fully rebuild build itself successfully, and the reciprocal version dependencies should match.

See gcc portion of patchset at http://lists.busybox.net/pipermail/buildroot/2017-August/200946.html for gcc-4.9.4 fixes including ucontext -> ucontext_t breakage.