Add ports for an i386 external GCC toolchain.

• devel/binutils: Remove i386 a.out linker scripts when building i386-binutils or a base/binutils that targets i386.
• devel/powerpc64-gcc: Remove float.h on i386 since it conflicts with sys/x86/include/float.h.
• devel/i386-{binutils,gcc,xtoolchain}: New ports.

Reviewed by: bapt (previous version)
Differential Revision: https://reviews.freebsd.org/D16228