add FLT_TRUE_MIN, etc. macros from C11

there was some question as to how many decimal places to use, since
one decimal place is always sufficient to identify the smallest
denormal uniquely. for now, I'm following the example in the C
standard which is consistent with the other min/max macros we already
had in place.

arch/mips/bits/float.h

@@ -1,6 +1,7 @@
 #define FLT_ROUNDS 1
 #define FLT_EVAL_METHOD 0
 
+#define LDBL_TRUE_MIN 4.9406564584124654e-324
 #define LDBL_MIN 2.2250738585072014e-308
 #define LDBL_MAX 1.7976931348623157e+308
 #define LDBL_EPSILON 2.2204460492503131e-16