#3299 Merge gcc and clang into a cc linter

Users can easily be confused when they set some options for a C or C++
compiler, and another compiler is run with different options, which
still reports errors. To remedy this, the existing `gcc` and `clang`
linters have been replaced with a `cc` linter that will run either
compiler.

This is a breaking change for ALE v3.0.0.

doc/ale-supported-languages-and-tools.txt

@@ -48,7 +48,7 @@ Notes:
 * C
   * `astyle`
   * `ccls`
-  * `clang`
+  * `clang` (`cc`)
   * `clangd`
   * `clang-format`
   * `clangtidy`!!
@@ -56,7 +56,7 @@ Notes:
   * `cpplint`!!
   * `cquery`
   * `flawfinder`
-  * `gcc`
+  * `gcc` (`cc`)
   * `uncrustify`
 * C#
   * `csc`!!
@@ -66,7 +66,7 @@ Notes:
 * C++ (filetype cpp)
   * `astyle`
   * `ccls`
-  * `clang`
+  * `clang` (`cc`)
   * `clangcheck`!!
   * `clangd`
   * `clang-format`
@@ -76,7 +76,7 @@ Notes:
   * `cpplint`!!
   * `cquery`
   * `flawfinder`
-  * `gcc`
+  * `gcc` (`cc`)
   * `uncrustify`
 * Chef
   * `cookstyle`