Build: Restore -fshort-wchar flag for toolchain config

Removing the -fshort-wchar flag will cause link error with
RTX library while using armclang and debug mode.
This patch reverts changes in the commit
8bebd05745a8b27dccc6403f0215fa6e39de3bc1.

Change-Id: Iff165aa8b080d02f8fcfad07703ed3a274f346b0
Signed-off-by: Mingyang Sun <mingyang.sun@arm.com>
diff --git a/toolchain_ARMCLANG.cmake b/toolchain_ARMCLANG.cmake
index db53372..94fe9db 100644
--- a/toolchain_ARMCLANG.cmake
+++ b/toolchain_ARMCLANG.cmake
@@ -48,6 +48,7 @@
             -ffunction-sections
             -fno-builtin
             -fshort-enums
+            -fshort-wchar
             -funsigned-char
             -masm=auto
             -nostdlib
@@ -69,6 +70,7 @@
             $<$<COMPILE_LANGUAGE:C>:-ffunction-sections>
             $<$<COMPILE_LANGUAGE:C>:-fno-builtin>
             $<$<COMPILE_LANGUAGE:C>:-fshort-enums>
+            $<$<COMPILE_LANGUAGE:C>:-fshort-wchar>
             $<$<COMPILE_LANGUAGE:C>:-funsigned-char>
             $<$<COMPILE_LANGUAGE:C>:-masm=auto>
             $<$<COMPILE_LANGUAGE:C>:-nostdlib>