Drop builtin float.h for amd64-gcc.

GCC's builtin <float.h> header is not compatible with
sys/x86/include/float.h. Drop the builtin header for now. If at
some point GCC's notion of floating point constants for i386 can
converge with sys/x86/include/float.h this can be restored.

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D16073