libm: fix powf testcase failures
libm: fix powf testcase failures

Fixed failures:
Failure: Test: pow (1, NaN) == 1
Failure: Test: pow (1, inf) == 1
Failure: Test: pow (-1, inf) == 1
Failure: Test: pow (1, -inf) == 1
Failure: Test: pow (-1, -inf) == 1

Signed-off-by: Denys Vlasenko <42afb3a19136e88700e8b77002ea6a5a94c16782@googlemail.com>

joel-porquet/tsar-uclibc,joel-porquet/tsar-uclibc,joel-porquet/tsar-uclibc,joel-porquet/tsar-uclibc