[powerpc] fe{en,dis}ableexcept optimize bit translations

The exceptions passed to fe{en,dis}ableexcept() are defined in the ABI
as a bitmask, a combination of FE_INVALID, FE_OVERFLOW, etc.
Within the functions, these bits must be translated to/from the corresponding
enable bits in the Floating Point Status Control Register (FPSCR).
This translation is currently done bit-by-bit.  The compiler generates
a series of conditional bit operations.  Nicely, the "FE" exception
bits are all a uniform offset from the FPSCR enable bits, so the bit-by-bit
operation can instead be performed by a shift with appropriate masking.