tools/build: Work around broken Clang FreeBSD resource dir logic pre-13

Prior to Clang 13 (e.g. in the Clang 11 present in 13.0-RELEASE), the
resource directory logic for FreeBSD was broken and would not resolve
symlinks, meaning symlinks would only work if in a directory next to the
containing lib directory. Therefore we cannot even use a symlink for
worldtmp, we have to make a wrapper script that execs the real binary
via an absolute path.

Reported by: markj
Reviewed by: markj
Fixes: 65f28f63a73d ("tools/build: Create toolchain symlinks for non-absolute compiler/linker")
Differential Revision: https://reviews.freebsd.org/D41238