sim: Specify -fshort-wchar as NuttX wchar_t is 16-bit

* For C, wchar_t is provided by the OS. On NuttX, it's uint16_t.
  (Except wide character literals, which uses the compiler
   built-in knowledge of wchar_t.)

* For C++, wchar_t is a built-in type. It's provided by the compiler.

* The compilers built-in wchar depends on the compiler configuration.
  For the sim, the compilers are usually configured with wchar_t == int,
  which is 32-bit.

* wchar_t should be compatible between C and C++.

This commit fixes the mismatches by telling the compiler to use
16-bit wchar_t.

An alternative is to make NuttX use 32-bit wchar_t if the compiler
is configured with 32-bit wchar_t. It can increase the runtime
footprint. While it might be ok for the sim, it might be a problem
for real devices.

boards/sim/sim/sim/scripts/Make.defs

@@ -93,11 +93,16 @@ NM = $(CROSSDEV)nm
 OBJCOPY = $(CROSSDEV)objcopy
 OBJDUMP = $(CROSSDEV)objdump
 
+# Note: -fshort-wchar for the case where NuttX and the host OS have
+# differnt wchar_t. On Nuttx, it's uint16_t. On macOS, it's 32-bit.
 CFLAGS := $(ARCHWARNINGS) $(ARCHOPTIMIZATION) \
-   $(ARCHCPUFLAGS) $(ARCHINCLUDES) $(ARCHDEFINES) $(EXTRAFLAGS) -pipe
+   $(ARCHCPUFLAGS) $(ARCHINCLUDES) $(ARCHDEFINES) $(EXTRAFLAGS) -pipe \
+   -fshort-wchar
 CXXFLAGS := $(ARCHWARNINGSXX) $(ARCHOPTIMIZATION) \
-   $(ARCHCPUFLAGSXX) $(ARCHXXINCLUDES) $(ARCHDEFINES) $(EXTRAFLAGS) -pipe
-CPPFLAGS := $(ARCHINCLUDES) $(ARCHDEFINES) $(EXTRAFLAGS)
+   $(ARCHCPUFLAGSXX) $(ARCHXXINCLUDES) $(ARCHDEFINES) $(EXTRAFLAGS) -pipe \
+   -fshort-wchar
+CPPFLAGS := $(ARCHINCLUDES) $(ARCHDEFINES) $(EXTRAFLAGS) \
+   -fshort-wchar
 AFLAGS := $(CFLAGS) -D__ASSEMBLY__
 
 ifeq ($(CONFIG_LIBCXX),y)