How about the trivial solution below?

Add this somewhere widely included:
i.e. src/sys/arch/mips/include/cdefs.h

#if defined(__mipseb__) || defined(__mipsel_)
#define __mips__ 1
#endif

Noriyuki Soda writes:
 > [...]
 > In many case, "if defined(__mips__)" seems to be better than
 > "if defined(__mipseb__) || defined(__mipsel_)". If the code doesn't
 > concern with the endian, why don't just use "__mips__" ? 
 > The "__mips__" symbol is defined on all MIPS platforms, not only NetBSD,
 > thus we cannot remove -D__mips__ from gcc configuration, anyway.
 > 
 > The real problem seems to be that there is no symbol which defines CPU
 > architecure ("mips").
 > 
 > The "MACHINE" is needed because we have to distinct the platform.
 > The "MACHINE_ARCH" is needed because we have to distinct the binary
 > compatibility.
 > But there is no symbol which can be used to distinct the source level
 > compatibility. If such symbol exist, we can remove all hacks like
 > ${MACHINE_ARCH:C/mipse[bl]/mips/}.
 > 
 > There will be same problem on SH3. (SH3 runs both little and big endian).
 > --
 > soda