Make comments more consistent

- Changes most `/* comments */` to `// comments`
- Changes `/**` block comments consistently to `/*`
- Adds consistent license comments to all files

Also renames `T_POP_SET` to `T_Z80_SET`

include/link/output.h

@@ -6,7 +6,7 @@
  * SPDX-License-Identifier: MIT
  */
 
-/* Outputting the result of linking */
+// Outputting the result of linking
 #ifndef RGBDS_LINK_OUTPUT_H
 #define RGBDS_LINK_OUTPUT_H
 
@@ -14,22 +14,22 @@
 
 #include "link/section.h"
 
-/**
+/*
  * Registers a section for output.
  * @param section The section to add
  */
 void out_AddSection(struct Section const *section);
 
-/**
+/*
  * Finds an assigned section overlapping another one.
  * @param section The section that is being overlapped
  * @return A section overlapping it
  */
 struct Section const *out_OverlappingSection(struct Section const *section);
 
-/**
+/*
  * Writes all output (bin, sym, map) files.
  */
 void out_WriteFiles(void);
 
-#endif /* RGBDS_LINK_OUTPUT_H */
+#endif // RGBDS_LINK_OUTPUT_H