devel/llvm*: Fix wrapper scripts to support symlinking to them

The wrapper scripts currently do not resolve the real path to $0:

$ install -l rs /usr/local/bin/clang50 gcc
$ ./gcc
./gcc: /usr/local/llvm50/bin/gcc: not found

Here /usr/local/llvm50/bin/clang should be called not
/usr/local/llvm50/bin/gcc.

Approved by: brooks (maintainer)
Differential Revision: https://reviews.freebsd.org/D14727