malloc: cast size down to int in bin_index functions

even if size_t was 32-bit already, the fact that the value was
unsigned and that gcc is too stupid to figure out it would be positive
as a signed quantity (due to the immediately-prior arithmetic and
conditionals) results in gcc compiling the integer-to-float conversion
as zero extension to 64 bits followed by an "fildll" (64 bit)
instruction rather than a simple "fildl" (32 bit) instruction on x86.
reportedly fildll is very slow on certain p4-class machines; even if
not, the new code is slightly smaller.

src/malloc/malloc.c

@@ -120,14 +120,14 @@ static int bin_index(size_t x)
 	x = x / SIZE_ALIGN - 1;
 	if (x <= 32) return x;
 	if (x > 0x1c00) return 63;
-	return ((union { float v; uint32_t r; }){ x }.r>>21) - 496;
+	return ((union { float v; uint32_t r; }){(int)x}.r>>21) - 496;
 }
 
 static int bin_index_up(size_t x)
 {
 	x = x / SIZE_ALIGN - 1;
 	if (x <= 32) return x;
-	return ((union { float v; uint32_t r; }){ x }.r+0x1fffff>>21) - 496;
+	return ((union { float v; uint32_t r; }){(int)x}.r+0x1fffff>>21) - 496;
 }
 
 #if 0