[msan] Fix strxfrm test

Revert D83719 and explicitly set locate to "C".

Author: vitalybuka

compiler-rt/test/msan/strxfrm.cpp

@@ -7,12 +7,12 @@
 #include <string.h>
 
 int main(void) {
-  char q[30];
+  char q[10];
   size_t n = strxfrm(q, "abcdef", sizeof(q));
   assert(n < sizeof(q));
   __msan_check_mem_is_initialized(q, n + 1);
 
-  locale_t loc = newlocale(LC_ALL_MASK, "", (locale_t)0);
+  locale_t loc = newlocale(LC_ALL_MASK, "C", (locale_t)0);
 
   __msan_poison(&q, sizeof(q));
   n = strxfrm_l(q, "qwerty", sizeof(q), loc);