[TSAN] Fix infinite loop on targets where char is unsigned

For targets where char is unsigned (like PowerPC), something like
char c = fgetc(...) will never produce a char that will compare
equal to EOF so this loop does not terminate.
Change the type to int (which appears to be the POSIX return type
for fgetc).
This allows the test case to terminate normally on PPC.

compiler-rt/test/tsan/fiber_cleanup.cpp

@@ -13,7 +13,7 @@ long count_memory_mappings() {
 
   FILE *proc_file = fopen(proc_file_name, "r");
   long line_count = 0;
-  char c;
+  int c;
   do {
     c = fgetc(proc_file);
     if (c == '\n') {