lang/rust: Use LLVM 3.6 from Ports instead of the bundled one

Dependencies are updated: Perl is not a dependency at all and Python is
a build dependency only.

Rust may fail to build if a previous version is already installed
because the bootstrapped rustc puts "-L/usr/local/lib" at the beginning
of the linker flags. Therefore, mark rust as a CONFLICTS_BUILD with
itself.

Fix the build as a normal user by setting permissions on all directories
to 0755 [1].

Be closer to the standalone installer's file hierarchy by installing
files such as "components", "manifest-rustc" and "uninstall.sh".

Add "files/patch-src_librustcback_target_freebsdbase.rs"
to remove "-L/usr/local/lib", "-L/usr/local/lib/gcc46" and
"-L/usr/local/lib/gcc44" from the builtin linker flags.

Differential Revision: https://reviews.freebsd.org/D2466
Submitted by: Chris Hutchinson [1]
Reviewed by: jonathan, kwm
Approved by: jonathan