Ben Gamari authored Sep 24, 2019 and Marge Bot committed Jan 07, 2020

Also refactor FP_GCC_EXTRA_FLAGS in a few ways:

 * We no longer support compilers which lack support for -fno-builtin
   and -fwrapv so remove the condition on GccVersion
 * These flags are only necessary when using the via-C backend
   so make them conditional on Unregisterised.

Fixes #15742.