Remove redundant extern keywords from function declarations/definitions. (#1495)

Extern keywords on function declarations/definitions provide no additional value since function declarations/definitions default to external linkage in C99, e.g. removing them won't change the semantics of the program.

The extern keywords were essentially a legacy from the early days of the project. This commit cleans this up across the whole codebase in one go to minimize history disruption.

The bulk of the changes in this commit were produced by a custom clang-tidy checker.

Note that variables declarations carrying the extern keyword are untouched by this commit since there the presence of the keyword actually has an impact on the semantics of the program.

JerryScript-DCO-1.0-Signed-off-by: Tilmann Scheller t.scheller@samsung.com

jerry-core/jrt/jrt.h

@@ -81,8 +81,8 @@
   enum { JERRY_STATIC_ASSERT_GLUE (static_assertion_failed_, __LINE__, msg) = 1 / (!!(x)) }
 
 #ifndef JERRY_NDEBUG
-extern void __noreturn jerry_assert_fail (const char *, const char *, const char *, const uint32_t);
-extern void __noreturn jerry_unreachable (const char *, const char *, const uint32_t);
+void __noreturn jerry_assert_fail (const char *, const char *, const char *, const uint32_t);
+void __noreturn jerry_unreachable (const char *, const char *, const uint32_t);
 
 #define JERRY_ASSERT(x) \
   do \
@@ -114,7 +114,7 @@ extern void __noreturn jerry_unreachable (const char *, const char *, const uint
 /**
  * Exit on fatal error
  */
-extern void __noreturn jerry_fatal (jerry_fatal_code_t);
+void __noreturn jerry_fatal (jerry_fatal_code_t);
 
 /*
  * Logging