add thiscall abi

Author: beepster4096

src/items/external-blocks.md

@@ -91,6 +91,8 @@ There are also some platform-specific ABI strings:
 * `extern "vectorcall"` -- The `vectorcall` ABI -- corresponds to MSVC's
   `__vectorcall` and clang's `__attribute__((vectorcall))`
 * `extern "efiapi"` -- The ABI used for [UEFI] functions.
+* `extern "thiscall"` -- The default for C++ member functions on MSVC -- corresponds to MSVC's
+  `__thiscall` and GCC and clang's `__attribute__((thiscall))`
 
 ## Variadic functions