powerpc: Fix feraiseexcept and feclearexcept macros

A recent change to fenvinline.h modified the check if __e is a
a power of 2 inside feraiseexcept and feclearexcept macros.  It
introduced the use of the powerof2 macro but also removed the
if statement checking whether __e != 0 before issuing an mtfsb*
instruction.  This is problematic because powerof2 (0) evaluates
to 1 and without the removed if __e is allowed to be 0 when
__builtin_clz is called.  In that case the value 32 is passed
to __MTFSB*, which is invalid.

This commit uses __builtin_popcount instead of powerof2 to fix this
issue and avoid the extra check for __e != 0.  This was the approach
used by the initial versions of that previous patch.

Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>