revert r346588 for now

The rewrite of strcmp in assembly uses an instruction added in PowerISA
2.05, making it SIGILL on CPUs older than the POWER6, such as the PPC970 in
the PowerMac G5. Revert this until we get clang+lld, or retire the in-tree
binutils in favor of newer binutils with IFUNC support, whichever comes
first.