Draft: Adding -rpath to GHC

Introduces a list flag `-rpath` for users to specify runtime library
search paths where GHC's linker invocations will look for libraries at
load time.

Previously, clients (notably Cabal) would add specify runtime search
paths directly to the linker via @-optl-Wl,-rpath,/actual/path@,
bypassing the compiler abstraction and the special logic GHC has to
handle runtime search paths -- that is especially important on macOS (4ff9329224).

Additionally, this enables a better solution for cabal#7339, for the
above reasons.

@bgamari I'd appreciate if you could double check I'm doing the right thing in loadCmdLineLibs''. I've also added a TODO:QUESTION for something I don't know. Missing a test case and release notes.

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• add a [testcase to the testsuite][adding test].
• updates the users guide if applicable
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