Merge branch 'feature/cplusplus' into 'master'

C++ support

This change adds necessary support for compiling C++ programs:
- linking against libstdc++
- implementation of static initialization guards using FreeRTOS primitives: since we don't have condition variables at our disposal, and we don't want to allocate a synchronization primitive for every guard variable generated by the compiler, we imitate condition variables using a combination of a mutex, counting semaphore, and a counter (based on [Microsoft Research paper](https://www.microsoft.com/en-us/research/wp-content/uploads/2004/12/ImplementingCVs.pdf), albeit because we don't need *arbitrary* code to use these CVs, implementation gets simpler).

Note that libstdc++ also contains an implementation of `__cxa_guard_{acquire,release,abort}` functions. These implementations come from an `#ifndef GXX_THREADS` branch, i.e. are not aware of multthreading. There are three ways of replacing these libstdc++ functions with our implementation:

1. Move our code into gcc. Pros: cleanest solution. Cons: Such changes are unlikely to be merged by any upstream, so we end up maintaining our own forks of {gcc,crosstool-ng}.
2. Use library as it is built by crosstool, use `ar` to delete one object file (`guards.o`), add this library to ESP-IDF. Pros: easy to implement. Cons: libstdc++ is a 15MB binary 😯 
3. Keep using libstdc++ from crosstool, force our implementation to be linked using a `-u` linker flag. Pros: no impact on repo size, easy to implement. Cons: somewhat less clean than 1 (and about as hacky as 2).

For the reasons mentioned, option (3) looks like the best tradeoff.

Ref. TW6702

See merge request !364

components/esp32/abi.cpp

@@ -1,114 +0,0 @@
-// Copyright 2015-2016 Espressif Systems (Shanghai) PTE LTD
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-
-//     http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-#include <stdlib.h>
-#include <assert.h>
-#include <cxxabi.h>
-#include <stdint.h>
-
-// using __cxxabiv1::__guard;
-
-void *operator new(size_t size)
-{
-    return malloc(size);
-}
-
-void *operator new[](size_t size)
-{
-    return malloc(size);
-}
-
-void operator delete(void * ptr)
-{
-    free(ptr);
-}
-
-void operator delete[](void * ptr)
-{
-    free(ptr);
-}
-
-extern "C" void __cxa_pure_virtual(void) __attribute__ ((__noreturn__));
-extern "C" void __cxa_deleted_virtual(void) __attribute__ ((__noreturn__));
-
-void __cxa_pure_virtual(void)
-{
-    abort();
-}
-
-void __cxa_deleted_virtual(void)
-{
-    abort();
-}
-
-#if 0
-typedef struct {
-    uint8_t guard;
-    uint8_t ps;
-} guard_t;
-
-extern "C" int __cxa_guard_acquire(__guard* pg)
-{
-    uint8_t ps = xt_rsil(15);
-    if (reinterpret_cast<guard_t*>(pg)->guard) {
-        xt_wsr_ps(ps);
-        return 0;
-    }
-    reinterpret_cast<guard_t*>(pg)->ps = ps;
-    return 1;
-}
-
-extern "C" void __cxa_guard_release(__guard* pg)
-{
-    reinterpret_cast<guard_t*>(pg)->guard = 1;
-    xt_wsr_ps(reinterpret_cast<guard_t*>(pg)->ps);
-}
-
-extern "C" void __cxa_guard_abort(__guard* pg)
-{
-    xt_wsr_ps(reinterpret_cast<guard_t*>(pg)->ps);
-}
-#endif
-
-extern "C" void __cxa_throw_bad_array_new_length()
-{
-    abort();
-}
-
-namespace std
-{
-void __throw_bad_function_call()
-{
-    abort();
-}
-
-void __throw_length_error(char const*)
-{
-    abort();
-}
-
-void __throw_bad_alloc()
-{
-    abort();
-}
-
-void __throw_logic_error(const char* str)
-{
-    abort();
-}
-
-void __throw_out_of_range(const char* str)
-{
-    abort();
-}
-}