[build] Add LLVM_LOCAL_RPATH which can set an rpath on just unit test binaries

After D80096, bots that build clang for distribution and that can't use
system gcc / libstdc++ need to pass a working rpath so that unit test
binaries can run. The method suggested in GettingStarted.rst works fine
for local development, but it results in an absolute local rpath ending
up even in distributed binaries like clang, which is both ugly and
unnecessary.

Add an explicit toggle that can be used to add an rpath only for the
non-distributed binaries that need it.

Differential Revision: https://reviews.llvm.org/D80534

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