Use of literal "ar" in bld/Makefile

[Paul Hargrove]

Currently bld/Makefile uses ar rcs to construct libupcxx.a from the *.o files. Though GASNet configure probes for a $(AR) for use in makefiles, UPC++ is not using it.

I was somewhat surprised that this has not shown up as an issue with cross-compilation.
However, it did show itself to be an issue when I tried LLVM's LTO support, because doing so requires use of llvm-ar in place of the system ar.

For completeness, I should note that my attempts to use LLVM's LTO support with UPC++ appeared to encounter additional issues. So, to the best of my knowledge, resolving this issue would not be sufficient to apply LLVM's LTO to UPC++ applications.