Use useful names for Symbol Addr and Names in Linker.c

Replace `char*` and `void*` with `SymbolName` and `SymbolAddr` in
`Linker.c`. Gives some useful information about what the variables are
used for and also normalizes the types used in Mac, Linux and Windows

Test Plan:
./validate on all platforms that use the runtime linker.

For unix platforms please ensure `DYNAMIC_GHC_PROGRAMS=NO` is
 added to your validate file.

This is a continuation from D2184

Reviewers: austin, erikd, simonmar, bgamari

Reviewed By: bgamari

Subscribers: thomie, #ghc_windows_task_force

Differential Revision: https://phabricator.haskell.org/D2250

GHC Trac Issues: #11816

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