This removes the hardcoded FreeBSD OS version used in target triple.
Also fixes wrong TARGET_TRIPLE assembly where TARGET_ABI is written where "vendor" string is expected. It's expected to be in the form "<arch><sub>-<vendor>-<sys>-<abi>" , but it's currently calculated as "<arch><sub>-<abi>-<sys>"
As example, 'make buildworld TARGET=powerpc TARGET_ARCH=powerpc64 TARGET_ABI=elfv2' must end like: 'TARGET_TRIPLE=powerpc64-unknown-freebsd13.0-elfv2'
TARGET_TRIPLE is used as argument to clang's "-target" parameter specially when cross compiling.
*note <sub> is ignored in this patch as well