rts: avoid building unused cbits on wasm

This patch avoids building rts cbits that aren't used when targetting wasm. The main motivation is getting rid of spurious LTO-specific link-time error messages when building the rts with LLVM LTO mode.

rts/RtsStartup.c

@@ -347,8 +347,10 @@ hs_init_ghc(int *argc, char **argv[], RtsConfig rts_config)
     /* Initialise the adjustors subsystem */
     initAdjustors();
 
+#if RTS_LINKER_USE_MMAP
     /* Initialise mmapForLinker */
     initLinkerMMap();
+#endif
 
     /* Initialise the stats department, phase 1 */
     initStats1();