Define wchar_t according to __WCHAR_TYPE__ (#1431)

b/298211986
b/246854012

Windows platform modular builds don't work with the current definition
of wchar_t for c files.

Setting wchar_t according to the predefined C macro __WCHAR_TYPE__
ensures that wchar_t is defined correctly.

Error shows up here: b/246854012#comment41 and was discussed in
go/lbreview/260401

(cherry picked from commit 474b3db)

third_party/musl/arch/aarch64/bits/alltypes.h

@@ -11,10 +11,14 @@
 #define __LONG_MAX 0x7fffffffffffffffL
 
 #ifndef __cplusplus
+#if defined(USE_COBALT_CUSTOMIZATIONS)
+typedef __WCHAR_TYPE__ wchar_t;
+#else 
 #if defined(__NEED_wchar_t) && !defined(__DEFINED_wchar_t)
 typedef unsigned wchar_t;
 #define __DEFINED_wchar_t
 #endif
+#endif // defined(USE_COBALT_CUSTOMIZATIONS)
 
 #endif
 #if defined(__NEED_wint_t) && !defined(__DEFINED_wint_t)

third_party/musl/arch/arm/bits/alltypes.h

@@ -12,10 +12,14 @@
 #define __LONG_MAX 0x7fffffffL
 
 #ifndef __cplusplus
+#if defined(USE_COBALT_CUSTOMIZATIONS)
+typedef __WCHAR_TYPE__ wchar_t;
+#else 
 #if defined(__NEED_wchar_t) && !defined(__DEFINED_wchar_t)
 typedef unsigned wchar_t;
 #define __DEFINED_wchar_t
 #endif
+#endif // defined(USE_COBALT_CUSTOMIZATIONS)
 
 #endif
 

third_party/musl/arch/x86_64/bits/alltypes.h

@@ -1,16 +1,18 @@
 #define _Addr long
 #define _Int64 long
 #define _Reg long
-
 #define __BYTE_ORDER 1234
 #define __LONG_MAX 0x7fffffffffffffffL
 
 #ifndef __cplusplus
+#if defined(USE_COBALT_CUSTOMIZATIONS)
+typedef __WCHAR_TYPE__ wchar_t;
+#else 
 #if defined(__NEED_wchar_t) && !defined(__DEFINED_wchar_t)
 typedef int wchar_t;
 #define __DEFINED_wchar_t
 #endif
-
+#endif // defined(USE_COBALT_CUSTOMIZATIONS)
 #endif
 
 #if defined(__FLT_EVAL_METHOD__) && __FLT_EVAL_METHOD__ == 2