get in the bug fix for pow, from fdlibm 5.3. also adapt it for

get in the bug fix for pow, from fdlibm 5.3.  also adapt it for
powf, the float version of it
>     1. e_pow.c incorrect results when
>         x is very close to -1.0 and y is very large, e.g.
>         pow(-1.0000000000000002e+00,4.5035996273704970e+15) = 0
>         pow(-9.9999999999999978e-01,4.5035996273704970e+15) = 0
>         Correct results are close to -e and -1/e.
while here merge more changes for pow, fixes sign in oflow/uflow
cases
ok millert@