Support for generic GCC toolchains

Type: LanguageService

Describe the bug

• OS and Version: Windows 10 20H2
• VS Code Version: 1.52.1
• C/C++ Extension Version: 1.2.0-insiders
• Does this issue involve using SSH remote to run the extension on a remote machine?: No
• A clear and concise description of what the bug is, including information about the workspace (i.e. is the workspace a single project or multiple projects, size of the project, etc).

Many different embedded platforms use GCC as their compiler. This presents a problem for the extension, since it only supports x86, x64, arm and arm64, while there are many embedded platforms (just to name a few: AVR and ESP32). It's impossible to keep track of them all, so we should create a generic GCC target, which will only do the minimal things to get IntelliSense working (get C and C++ standards, get include folders, get predefined macros, etc.)

Steps to reproduce

Create a C++ configuration with a compiler that targets an embedded platform.

Expected behavior

Proper IntelliSense for embedded toolchains.

cpptools/didChangeCppProperties
Attempting to get defaults from C++ compiler in "compilerPath" property: 'C:/Users/nghuu/.platformio/packages/toolchain-atmelavr/bin/avr-gcc.exe'
Querying compiler for default C++ language standard using command line: "C:/Users/nghuu/.platformio/packages/toolchain-atmelavr/bin/avr-gcc.exe" -x c++ -E -dM nul
Querying compiler for default C language standard using command line: "C:/Users/nghuu/.platformio/packages/toolchain-atmelavr/bin/avr-gcc.exe" -x c -E -dM nul
Querying compiler's default target using command line: "C:/Users/nghuu/.platformio/packages/toolchain-atmelavr/bin/avr-gcc.exe" -dumpmachine
Compiler returned default target value: avr

Unhandled default compiler target value detected: avr

Attempting to get defaults from compiler found on the machine: 'C:\llvm-mingw\bin\gcc.exe'
Querying compiler for default C++ language standard using command line: "C:\llvm-mingw\bin\gcc.exe" -x c++ -E -dM nul
Querying compiler for default C language standard using command line: "C:\llvm-mingw\bin\gcc.exe" -x c -E -dM nul
Querying compiler's default target using command line: "C:\llvm-mingw\bin\gcc.exe" -dumpmachine
Unhandled default compiler target value detected: 
No suitable compiler found. Please set the "compilerPath" in c_cpp_properties.json.
Attempting to get defaults from C compiler in "compilerPath" property: 'C:/Users/nghuu/.platformio/packages/toolchain-atmelavr/bin/avr-gcc.exe'
Attempting to get defaults from compiler found on the machine: 'C:\llvm-mingw\bin\gcc.exe'
No suitable compiler found. Please set the "compilerPath" in c_cpp_properties.json.

Screenshots

Unable to locate avr include files because the default include paths aren't set up properly:

Additional context

Default search paths for avr-gcc:

install: c:\users\nghuu\.platformio\packages\toolchain-atmelavr\bin\../lib/gcc/avr/5.4.0/
programs: =c:/users/nghuu/.platformio/packages/toolchain-atmelavr/bin/../libexec/gcc/avr/5.4.0/;c:/users/nghuu/.platformio/packages/toolchain-atmelavr/bin/../libexec/gcc/;c:/users/nghuu/.platformio/packages/toolchain-atmelavr/bin/../lib/gcc/avr/5.4.0/../../../../avr/bin/avr/5.4.0/;c:/users/nghuu/.platformio/packages/toolchain-atmelavr/bin/../lib/gcc/avr/5.4.0/../../../../avr/bin/
libraries: =c:/users/nghuu/.platformio/packages/toolchain-atmelavr/bin/../lib/gcc/avr/5.4.0/;c:/users/nghuu/.platformio/packages/toolchain-atmelavr/bin/../lib/gcc/;c:/users/nghuu/.platformio/packages/toolchain-atmelavr/bin/../lib/gcc/avr/5.4.0/../../../../avr/lib/avr/5.4.0/;c:/users/nghuu/.platformio/packages/toolchain-atmelavr/bin/../lib/gcc/avr/5.4.0/../../../../avr/lib/

The include paths are there, but the extension isn't grabbing it.

Start with compilerPath handling in c_cpp_properties.json and the C/C++: Log Diagnostics output, especially -dumpmachine target detection. Trace how the avr target reaches “Unhandled default compiler target value detected”; done means generic GCC targets provide standards, include paths, and predefined macros for embedded toolchains, with proper IntelliSense.