compiler: handle >32bit exponent in Ldexp

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