Cleanup a few cases where isinf is used to get the signbit to improve the readability...

Cleanup a few cases where isinf is used to get the signbit to improve the readability and maintainability and allow inlining.

2015-09-18  Wilco Dijkstra  <wdijkstr@arm.com>

        * math/w_tgamma.c (__ieee754_gamma_r): Use explicit sign check.
        * math/w_tgammaf.c (__ieee754_gammaf_r): Likewise.
        * math/w_tgammal.c (__ieee754_gammal_r): Likewise.
        * stdio-common/printf_fp.c (___printf_fp):
        Use signbit to get the sign. Use isinf macro to allow inlining.
        * stdio-common/printf_fphex.c (__printf_fphex): Likewise.
        * stdio-common/printf_size.c (__printf_size): Likewise.