math: fix undefined shift in logf

A signed int shift overflowed when computing a constant mask, use hex
literal instead.  This is unlikely to cause actual issues unless the
code was compiled with ubsan or similar instrumentation specifically
to catch this. The stripped libc.so is unchanged on x86_64.
Reported by q66 on irc.

src/math/logf.c

@@ -53,7 +53,7 @@ float logf(float x)
 	tmp = ix - OFF;
 	i = (tmp >> (23 - LOGF_TABLE_BITS)) % N;
 	k = (int32_t)tmp >> 23; /* arithmetic shift */
-	iz = ix - (tmp & 0x1ff << 23);
+	iz = ix - (tmp & 0xff800000);
 	invc = T[i].invc;
 	logc = T[i].logc;
 	z = (double_t)asfloat(iz);