lang/rust: Fix ${PORT_LLVM_BUILD_DEPENDS}

The BUILD_DEPENDS argument for the PORT_LLVM option was not updated: it
was using the ${LLVM_PREFIX} variable which was removed. Now, it uses
${LOCALBASE} and ${LLVM_VER} as a suffix to the executable verified.

Submitted by: Rainer Hurling <rhurlin@gwdg.de>
Reported by: Rainer Hurling <rhurlin@gwdg.de>