fix definitions of FP_ILOGB constants

two issues: (1) the type was wrong (unsigned instead of signed int),
and (2) the value of FP_ILOGBNAN should be INT_MIN rather than INT_MAX
to match the ABI. this is also much more useful since INT_MAX
corresponds to a valid input (infinity). the standard would allow us
to set FP_ILOGB0 to -INT_MAX instead of INT_MIN, which would give us
distinct values for ilogb(0) and ilogb(NAN), but the benefit seems way
too small to justify ignoring the ABI.

note that the macro is just a "portable" (to any twos complement
system where signed and unsigned int have the same width) way to write
INT_MIN without needing limits.h. it's valid to use this method since
these macros are not required to work in #if directives.

include/math.h

@@ -28,8 +28,8 @@ extern "C" {
 #define MATH_ERREXCEPT 2
 #define math_errhandling 2
 
-#define FP_ILOGBNAN (((unsigned)-1)>>1)
-#define FP_ILOGB0 (~FP_ILOGBNAN)
+#define FP_ILOGBNAN (-1-(int)(((unsigned)-1)>>1))
+#define FP_ILOGB0 FP_ILOGBNAN
 
 #define FP_NAN       0
 #define FP_INFINITE  1