Complete the namespace dance

Add missing namespace.h include

provide a common softfloat fenv implemenation and use it for softfloat builds.
restore ABI compatibility with previous releases for ieeefp.h on sh3.
add namespace.h protection for all the fenv interf

provide a common softfloat fenv implemenation and use it for softfloat builds.
restore ABI compatibility with previous releases for ieeefp.h on sh3.
add namespace.h protection for all the fenv interfaces.
use MKSOFTFLOAT on sh3 instead of assuming softfloat.
standardize on comparing MKSOFTFLOAT with "no".
remove the arm-specific softfloat fenv code (which also had several bugs).
fix logic errors in the arm hardfloat feraiseexcept() and feupdateenv().

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Fix exception status vs mask confusion, of course feenableexcept(),
fedisableexcept() and fegetexcept() all operate on the mask.

Add missing return in __SOFTFP__ variant, pointed out by htodd.

Add our common non standard extensions and fix typo (FE_EXCEPT_ALL ->
FE_ALL_EXCEPT).

Fix feclearexcept return value (should be 0).

Make this compile for earmhf

Add a C99 <fenv.h> implementation for arm.