compiler: handle >32bit exponent in Ldexp

Libgo's implementation of math.Ldexp declared the libc "ldexp" as
    taking an 'int' exponent argument, which is not quite right for 64-bit
    platforms (exp arg is always int32); this could yield incorrect
    results for exponent values outside the range of Minint32/Maxint32.
    Fix by upating the type for the libc version of ldexp, and adding
    guards to screen for out-of-range exponents.
    
    Fixes #21323.
    
    Reviewed-on: https://go-review.googlesource.com/54250

From-SVN: r250992

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-db685a1a9aa8b3b916dd6d1284895e01d73158e1
+5fd112e5c2968e94761c41519c451d789e23a92b
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/math/ldexp.go

@@ -13,10 +13,15 @@ package math
 //	Ldexp(NaN, exp) = NaN
 
 //extern ldexp
-func libc_ldexp(float64, int) float64
+func libc_ldexp(float64, int32) float64
 
 func Ldexp(frac float64, exp int) float64 {
-	r := libc_ldexp(frac, exp)
+	if exp > MaxInt32 {
+		exp = MaxInt32
+	} else if exp < MinInt32 {
+		exp = MinInt32
+	}
+	r := libc_ldexp(frac, int32(exp))
 	return r
 }