Remove availability definitions for mbs APIs.

Definitions for these are provided in libandroid_support for API
levels that do not expose this in the stubs. For the rare cases where
libandroid_support is not being used this will result in a lower
quality diagnostic (undefined reference instead of "not available
until API 21"), but other fixes would also have that behavior because
the libandroid_support headers are *always* available, even if
libandroid_support won't be linked.

Test: Reverted xfailed tests for #1108 and reran tests with this
Bug: android/ndk#1108
Change-Id: I371f5b9d7caeef8dc7c80f2f6d11280ecba119c9

libc/include/stdlib.h

@@ -161,11 +161,11 @@ const char* getprogname(void) __INTRODUCED_IN(21);
 void setprogname(const char* __name) __INTRODUCED_IN(21);
 
 int mblen(const char* __s, size_t __n) __INTRODUCED_IN_NO_GUARD_FOR_NDK(26);
-size_t mbstowcs(wchar_t* __dst, const char* __src, size_t __n) __INTRODUCED_IN(21) __VERSIONER_NO_GUARD;
+size_t mbstowcs(wchar_t* __dst, const char* __src, size_t __n) __INTRODUCED_IN_NO_GUARD_FOR_NDK(21);
 int mbtowc(wchar_t* __wc_ptr, const char* __s, size_t __n) __INTRODUCED_IN_NO_GUARD_FOR_NDK(21);
 int wctomb(char* __dst, wchar_t __wc) __INTRODUCED_IN_NO_GUARD_FOR_NDK(21);
 
-size_t wcstombs(char* __dst, const wchar_t* __src, size_t __n) __INTRODUCED_IN(21) __VERSIONER_NO_GUARD;
+size_t wcstombs(char* __dst, const wchar_t* __src, size_t __n) __INTRODUCED_IN_NO_GUARD_FOR_NDK(21);
 
 #if __ANDROID_API__ >= 21
 size_t __ctype_get_mb_cur_max(void) __INTRODUCED_IN(21);