Remove jerry-libc (#2332)

Rationale:
- There is no port under targets/ that would use it. All of them
  turn it off when building.
- That's no surprise, as jerry-libc supports no barebone MCUs but
  posix targets with syscalls only. Actually, that's Linux only,
  because macOS builds have turned off the use of jerry-libc a
  while ago.
- And there is no point in maintaining a highly restricted set of
  libc functions: as soon as someone wants to use JerryScript in a
  scenario that needs more functions than jerry-main, they have to
  choose a different libc (most problably the compiler's default
  one).

I think that we should not keep supporting an otherwise unused
library for the purposes of jerry-main on arm/x86/x64-linux  only.

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu

jerry-core/CMakeLists.txt

@@ -210,10 +210,6 @@ endif()
 
 # Enable debugger
 if(FEATURE_DEBUGGER)
-  if(JERRY_LIBC)
-    message(FATAL_ERROR "This configuration is not supported. Please build against your system libc to enable the JerryScript debugger.")
-  endif()
-
   set(DEFINES_JERRY ${DEFINES_JERRY} JERRY_DEBUGGER)
 endif()
 
@@ -241,10 +237,6 @@ else()
   message(FATAL_ERROR "Profile file: '${FEATURE_PROFILE}' doesn't exist!")
 endif()
 
-if(JERRY_LIBC AND FEATURE_SYSTEM_ALLOCATOR)
-  MESSAGE(FATAL_ERROR "This configuration is not supported. Please build against your system libc to enable the system allocator.")
-endif()
-
 # RegExp strict mode
 if(FEATURE_REGEXP_STRICT_MODE)
   set(DEFINES_JERRY ${DEFINES_JERRY} ENABLE_REGEXP_STRICT_MODE)
@@ -295,10 +287,6 @@ if(JERRY_LIBM)
   target_link_libraries(${JERRY_CORE_NAME} jerry-libm)
 endif()
 
-if(JERRY_LIBC)
-  target_link_libraries(${JERRY_CORE_NAME} jerry-libc)
-endif()
-
 separate_arguments(EXTERNAL_LINK_LIBS)
 foreach(EXT_LIB ${EXTERNAL_LINK_LIBS})
   target_link_libraries(${JERRY_CORE_NAME} ${EXT_LIB})