Improve gen-libm-test.pl LIT() application

When bootstrapping float128, this exposed a number of areas where
the L suffix is incorrectly applied to simple expressions when it
should be applied to each constant in the expression.

In order to stave off more macros in libm-test.inc, apply_lit is
made slightly more intelligent.  It will now split expressions
based on space characters, and attempt to apply LIT() to each
token.

Having done this, there are numerous spacing issues within
libm-test.inc which have been fixed.

The above is problematic when the L real suffix is not the most
expressive modifier, and the compiler complains (i.e ppc64) or
silently truncates a value (i.e ppc64).