libm: add i387/ dir, fix test51 compilation

. if $MACHINE_ARCH is unknown, use $MACHINE like in libc
	. fixes test51 compilation for gcc/clang (with other compile
	  and link fixes)

lib/nbsd_libm/Makefile

@@ -13,6 +13,14 @@
 #
 #
 
+.warning  MACHINE_ARCH is first ${MACHINE_ARCH}
+
+.if ${MACHINE_ARCH} == "unknown"
+MACHINE_ARCH:= ${MACHINE}
+.endif
+
+.warning  MACHINE_ARCH is now ${MACHINE_ARCH}
+
 #
 # There are two options in making libm at fdlibm compile time:
 # 	_IEEE_LIBM 	--- IEEE libm; smaller, and somewhat faster

lib/nbsd_libm/arch/i387/fenv.c

@@ -122,8 +122,10 @@ static void __test_sse(void)
 	size_t oldlen = sizeof(__HAS_SSE);
 	int rv;
 
+#ifndef __minix
 	rv = sysctlbyname("machdep.sse", &__HAS_SSE, &oldlen, NULL, 0);
 	if (rv == -1)
+#endif
 		__HAS_SSE = 0;
 }