Add support for amd64 and i386 to base/binutils and base/gcc.

[jhb]

Summary

• To support amd64, map amd64 to x86_64 when generating the --host argument to configure in bsd.port.mk and when generating --target arguments in port Makefiles.
• Add entries to the plists for i386 and amd64. amd64 generally includes both amd64-specific entries as well as i386 entries.
• Don't include the builtin float.h for i386 and amd64.

Test Plan

• have only built all four packages and verified check-plist is as close as it is for other architectures (like mips) (I have some pending fixes to cleanup check-plist that I will post in a future change)