[libc][test] -Wimplicit-fallthrough, -Wwrite-strings and non-GCC warnings (#124036)

* Enabled `-Wimplicit-fallthrough`
* Enabled `-Wwrite-strings`
* Enabled non-GCC warnings
* Fix non-GCC to mean `clang`
* Move `-Wstrict-prototypes` to common section

See:
#122835 (comment)

Relates to #119281

Author: vinay-deshmukh

libc/cmake/modules/LLVMLibCTestRules.cmake

@@ -38,9 +38,8 @@ function(_get_common_test_compile_options output_var c_test flags)
     endif()
     # list(APPEND compile_options "-Wconversion")
     # list(APPEND compile_options "-Wno-sign-conversion")
-    # list(APPEND compile_options "-Wimplicit-fallthrough")
-    # list(APPEND compile_options "-Wwrite-strings")
-    list(APPEND compile_options "-Wextra-semi")
+    list(APPEND compile_options "-Wimplicit-fallthrough")
+    list(APPEND compile_options "-Wwrite-strings")
     # Silence this warning because _Complex is a part of C99.
     if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
       if(NOT c_test)
@@ -51,13 +50,14 @@ function(_get_common_test_compile_options output_var c_test flags)
       list(APPEND compile_options "-Wno-gnu-imaginary-constant")
     endif()
     list(APPEND compile_options "-Wno-pedantic")
-    # if(NOT CMAKE_COMPILER_IS_GNUCXX)
-    #   list(APPEND compile_options "-Wnewline-eof")
-    #   list(APPEND compile_options "-Wnonportable-system-include-path")
-    #   list(APPEND compile_options "-Wstrict-prototypes")
-    #   list(APPEND compile_options "-Wthread-safety")
-    #   list(APPEND compile_options "-Wglobal-constructors")
-    # endif()
+    if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
+      list(APPEND compile_options "-Wstrict-prototypes")
+      list(APPEND compile_options "-Wextra-semi")
+      list(APPEND compile_options "-Wnewline-eof")
+      list(APPEND compile_options "-Wnonportable-system-include-path")
+      list(APPEND compile_options "-Wthread-safety")
+      # list(APPEND compile_options "-Wglobal-constructors")
+    endif()
   endif()
   set(${output_var} ${compile_options} PARENT_SCOPE)
 endfunction()