math/tomsfastmath: Miscellaneous refinements

• Use Clang on i386. Build succeeds with all supported FreeBSD versions.
• Switch to use release archive as per section 5.4.3 of Porter's Handbook.
• Remove post-patch target. It is no-op now as version 0.13.1 doesn't use "%cc" in inline assembly codes any more.
• Add LICENSE_FILE.
• Tidy up Makefile with portfmt.