fix: export RTS symbols for GHC API programs in dynamic builds

Programs that link against the ghc package and use features like Template
Haskell or GHCi need to export RTS symbols to dynamically loaded libraries.
When these programs load Haskell shared libraries via dlopen(), those
libraries reference RTS symbols like stg_INTLIKE_closure. Without exporting
these symbols from the executable, dlopen fails with "undefined symbol".

This fix adds platform-specific linker flags to GHC's static linker when
building executables that depend on the ghc package in dynamic mode:
- Linux/FreeBSD: -rdynamic (passes --export-dynamic to ld)
- macOS: -flat_namespace (makes all symbols visible across namespaces)
- Windows: not needed (--enable-auto-import already handles this)

This complements the similar fix for ghc-iserv (f8ca34d) and fixes
testsuite failures for tests like T5313, T8639_api, and dynCompileExpr
that use the GHC API with interpreterBackend.

See Note [Export dynamic symbols for GHC API programs] in
GHC/Linker/Static.hs and Note [ghc-iserv and dynamic symbol export]
in utils/ghc-iserv/ghc-iserv.cabal.in for full details.

Author: angerman

compiler/GHC/Linker/Static.hs

@@ -68,6 +68,24 @@ it is supported by both gcc and clang. Anecdotally nvcc supports
 -Xlinker, but not -Wl.
 -}
 
+{-
+Note [Export dynamic symbols for GHC API programs]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Programs linking against the ghc package need to export symbols from
+the RTS to dynamically loaded libraries. When running GHCi or Template
+Haskell, these programs load Haskell shared libraries via dlopen() that
+reference RTS symbols like stg_INTLIKE_closure. Without exporting these
+symbols from the executable, dlopen will fail with "undefined symbol".
+
+Platform-specific solutions:
+  Linux/FreeBSD: -rdynamic (passes --export-dynamic to ld)
+  macOS: -flat_namespace (makes all symbols visible across namespaces)
+  Windows: not needed (--enable-auto-import handles this)
+
+This is the same issue that ghc-iserv faces, and is documented in
+utils/ghc-iserv/ghc-iserv.cabal.in as Note [ghc-iserv and dynamic symbol export].
+-}
+
 linkBinary :: Logger -> TmpFs -> DynFlags -> ExecutableLinkMode -> UnitEnv -> [FilePath] -> [UnitId] -> IO ()
 linkBinary = linkBinary' False
 
@@ -252,6 +270,16 @@ linkBinary' staticLink logger tmpfs dflags blm unit_env o_files dep_units = do
                           then ["-Wl,--gc-sections"]
                           else [])
 
+                      -- See Note [Export dynamic symbols for GHC API programs]
+                      ++ (if ways_ `hasWay` WayDyn &&
+                             any ((== "ghc") . unitPackageNameString) pkgs
+                          then case platformOS platform of
+                                 os | osElfTarget os -> ["-rdynamic"]
+                                 OSDarwin -> ["-Wl,-flat_namespace"]
+                                 -- OSMinGW32 already has --export-all-symbols via --enable-auto-import
+                                 _ -> []
+                          else [])
+
                       ++ o_files
                       ++ lib_path_opts)
                       ++ extra_ld_inputs