Update README.md
local-patches: sync with xtensa gcc call0 ABI support

Fixed:
- possible a7/a15 corruption in functions with 7+ argument words and
  frame pointer enabled;
- vararg functions;
- setjmp/longjmp;
- nested functions;
- profiling with -pg option (_mcount call);

A bit improved code generation for SP manipulations.

Known broken:
- __builtin_return_address;
- __builtin_frame_address for frames other than current.

Unknown:
- C++ exception handling.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>