readme: Prefer the -Wl,--pdb= and -Wl,--pdb=<filename> forms for PDB output

Using two dashes for this option name is more idiomatic for
long GNU options (using only one dash only is supported for options
where it doesn't conflict with single-character options; it does
work for "-pdb" but "--pdb" is more correct).

Stop documenting the separate argument form "-Wl,--pdb,<filename>".
If noone would use this form, one could consider switching the tools
over to interpret "-Wl,--pdb" as a boolean option (optional_argument
instead of required_argument in getopt_long) as a less awkward syntax
for "-Wl,--pdb=".

README.md

@@ -170,10 +170,10 @@ changes:
 - Add `-gcodeview` to the compilation commands (e.g. in
   `wrappers/clang-target-wrapper.sh`), together with using `-g` as usual to
   enable debug info in general.
-- Add `-Wl,-pdb=` to linking commands. This creates a PDB file at the same
+- Add `-Wl,--pdb=` to linking commands. This creates a PDB file at the same
   location as the output EXE/DLL, but with a PDB extension. (By passing
-  `-Wl,-pdb=module.pdb` or `-Wl,-pdb,module.pdb` one can explicitly specify
-  the name of the output PDB file.)
+  `-Wl,--pdb=module.pdb` one can explicitly specify the name of the output
+  PDB file.)
 
 Even though LLVM supports this, there are some caveats with using it when
 building in MinGW mode; Microsoft debuggers might have assumptions about