Build + LibC: Enable -fstack-protector-strong in user space

Modify the user mode runtime to insert stack canaries to find stack corruptions.

The `-fstack-protector-strong` variant was chosen because it catches more
issues than vanilla `-fstack-protector`, but doesn't have substantial
performance impact like `-fstack-protector-all`.

Details:

    -fstack-protector enables stack protection for vulnerable functions that contain:

    * A character array larger than 8 bytes.
    * An 8-bit integer array larger than 8 bytes.
    * A call to alloca() with either a variable size or a constant size bigger than 8 bytes.

    -fstack-protector-strong enables stack protection for vulnerable functions that contain:

    * An array of any size and type.
    * A call to alloca().
    * A local variable that has its address taken.

Example of it catching corrupting in the `stack-smash` test:
```
courage ~ $ ./user/Tests/LibC/stack-smash
[+] Starting the stack smash ...
Error: Stack protector failure, stack smashing detected!
Shell: Job 1 (/usr/Tests/LibC/stack-smash) Aborted
```

1 files changed, 54 insertions, 0 deletions

diff --git a/Userland/Tests/LibC/stack-smash.cpp b/Userland/Tests/LibC/stack-smash.cpp
new file mode 100644
index 0000000000..00f6f44aa5
--- /dev/null
+++ b/Userland/Tests/LibC/stack-smash.cpp
@@ -0,0 +1,54 @@
+/*
+ * Copyright (c) 2021, Brian Gianforcaro <b.gianfo@gmail.com>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <cstdio>
+
+// Note: Needs to be 'noline' so stack canary isn't optimized out.
+static void __attribute__((noinline)) smasher(char* string)
+{
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Warray-bounds"
+    for (int i = 0; i < 256; i++) {
+        string[i] = 'A';
+    }
+#pragma GCC diagnostic pop
+}
+
+// Note: Needs to be 'noline' so stack canary isn't optimized out.
+static void __attribute__((noinline)) stack_to_smash()
+{
+    char string[8] = {};
+    smasher(string);
+}
+
+int main()
+{
+    puts("[+] Starting the stack smash...");
+    stack_to_smash();
+    puts("[+] Stack smash wasn't detected!");
+
+    return 0;
+}