Roughly equivalent to upstream 5f7cda29add367c5381775a314ec9f90e311457a.

--- a/boost/fiber/detail/cpu_relax.hpp
+++ b/boost/fiber/detail/cpu_relax.hpp
@@ -47,7 +47,7 @@ namespace detail {
 # else
 #  define cpu_relax() asm volatile ("nop" ::: "memory");
 # endif
-#elif BOOST_ARCH_MIPS
+#elif BOOST_ARCH_MIPS && (__mips_isa_rev > 1)
 # define cpu_relax() asm volatile ("pause" ::: "memory");
 #elif BOOST_ARCH_PPC
 // http://code.metager.de/source/xref/gnu/glibc/sysdeps/powerpc/sys/platform/ppc.h