The Gentoo folks caught a bug at https://bugs.gentoo.org/689162. The 689162 bug uses -march=bdver1 -msse4.1 on a AMD Bulldozer machine.
Investigating the issue we are missing the XOP header blake2b_simd.cpp. However, adding the XOP header is not enough for this particular config. Four source files fail to compile with the expected headers. We are waiting on the GCC folks to get back to us with a fix.