This patch fixes -Wundef warnings relating to __mips_isa_rev being
undefined.
Tested for mips64 (all three ABIs) that there is a clean build and
testsuite run with -Wno-error=undef removed (and my other -Wundef
patches applied).
* sysdeps/mips/dl-machine.h [__mips_isa_rev < 6]: Change
conditionals to [!defined __mips_isa_rev || __mips_isa_rev < 6].
* sysdeps/mips/machine-gmon.h [__mips_isa_rev < 6]: Likewise.
