newlib: libm: switch to autoconf long double macro

newlib: libm: switch to autoconf long double macro

Now that we require a recent version of autoconf, we can rely on this
macro working.  We shift the call in configure.ac down a little to
help keep the generated diff minimal -- there should be no functional
difference otherwise.  This is because the autoconf macros will call
a bunch of standard toolchain macros first, and arguably the current
code is incorrect in how it does its testing.