readme: Remove the note about PDB output not working for armv7/aarch64

PDB output now does work for armv7 (earlier it used to error out),
and the PDB output does work properly on aarch64.

(The recently added tests in run-lldb-tests.sh passes successfully on
aarch64. Debugging on armv7 with LLDB has got a few known issues though,
both with DWARF and with PDB, while WinDbg and/or Visual Studio might
work better.)

README.md

@@ -175,10 +175,6 @@ changes:
   `-Wl,-pdb=module.pdb` or `-Wl,-pdb,module.pdb` one can explicitly specify
   the name of the output PDB file.)
 
-Even though LLVM supports this, there are a few caveats with using it when
+Even though LLVM supports this, there are some caveats with using it when
-building in MinGW mode:
+building in MinGW mode; Microsoft debuggers might have assumptions about
-
+the C++ ABI used, which doesn't hold up with the Itanium ABI used in MinGW.
-- Microsoft debuggers might have assumptions about the C++ ABI used, which
-  doesn't hold up with the Itanium ABI used in MinGW.
-- This is unimplemented for the armv7 target, and while implemented for aarch64,
-  it doesn't seem to work properly there yet.