Define `Sudden_Underflow' when compiling for the Alpha

architecture, mainly to avoid getting a SIGFPE signal sent
when calling strtod(3) with certain input.

The SIGFPE has been sent because the code was not aware that
a Gradual Underflow is handled in software via traps on the
Alpha architecture, but is not implemented in our Alpha kernel
layer.

With `Sudden_Underflow' defined, strtod(3) should not depend
on Gradual Underflow and adjust its calculations accordingly,
which means that other, more subtle errors than the sending of
SIGFPE could be solved by this.

Discussed with:	bde
PR:		alpha/12623
PR:		alpha/17032
PR:		alpha/43567
MFC after:	7 days

lib/libc/stdlib/strtod.c

@@ -127,6 +127,9 @@ __FBSDID("$FreeBSD$");
 #else
 #define IEEE_LITTLE_ENDIAN
 #endif
+#if defined(__alpha__)
+#define	Sudden_Underflow
+#endif
 #endif /* defined(__i386__) ... */
 
 typedef	int32_t   Long;