Add detection of linker errors to gcc problem matcher

Environment:

• OS and Version: Windows 10
• VS Code Version: 1.74.3
• C/C++ Extension Version: v1.13.9
• If using SSH remote, specify OS of remote machine: CentOS 7.6

Bug Summary:

$gcc problem matcher does not match linker errors.
Trying to compile the example using standard g++ with -g flag will result in linker error with file and line info which should be matched.

To reproduce:

• Using the attached example below, run the default build task

tasks.json

{
    "version": "2.0.0",
    "tasks": [
        {
            "label": "Compile main file",
            "type": "shell",
            "command": "g++",
            "args": [
                "-g",
                "main.cpp"
            ],
            "problemMatcher":"$gcc",
            "presentation": {
                "reveal": "always",
                "revealProblems": "always"
            },
            "group": {
                "kind": "build",
                "isDefault": true
            }
        }
    ]
}

main.cpp

extern void external_func();

int main()
{
    external_func();
    return 0;
}

The output of the task when I run it on my machine is
/tmp/ccghmjfX.o: In function `main':
/home/karim/test/main.cpp:5: undefined reference to `external_func()'
collect2: error: ld returned 1 exit status

Expected behavior:

Linker error should be matched in the problems panel and highlighted in source file at line 5

Start with the supplied tasks.json and main.cpp, then run the default build task with the $gcc problem matcher to reproduce the linker output. Trace how the matcher handles the reported file and line, and verify completion when the linker error appears in the Problems panel and highlights main.cpp line 5.