[libc] Use atomic_load to read FILE->lock

FILE->lock is atomic_int and so should be read using atomic_load.
Previously this wasn't reported, but with a new Clang this is
triggering an error because of -Wtautological-unsigned-zero-compare.

Change-Id: I3fe11f4be356a96d688e90614a9498d3c9eba05e

third_party/ulib/musl/src/stdio/fgetc.c

@@ -2,7 +2,7 @@
 
 int fgetc(FILE* f) {
     int c;
-    if (f->lock < 0 || !__lockfile(f))
+    if (atomic_load(&f->lock) < 0 || !__lockfile(f))
         return getc_unlocked(f);
     c = getc_unlocked(f);
     __unlockfile(f);

third_party/ulib/musl/src/stdio/fputc.c

@@ -1,7 +1,7 @@
 #include "stdio_impl.h"
 
 int fputc(int c, FILE* f) {
-    if (f->lock < 0 || !__lockfile(f))
+    if (atomic_load(&f->lock) < 0 || !__lockfile(f))
         return putc_unlocked(c, f);
     c = putc_unlocked(c, f);
     __unlockfile(f);

third_party/ulib/musl/src/stdio/getc.c

@@ -2,7 +2,7 @@
 
 int getc(FILE* f) {
     int c;
-    if (f->lock < 0 || !__lockfile(f))
+    if (atomic_load(&f->lock) < 0 || !__lockfile(f))
         return getc_unlocked(f);
     c = getc_unlocked(f);
     __unlockfile(f);

third_party/ulib/musl/src/stdio/putc.c

@@ -1,7 +1,7 @@
 #include "stdio_impl.h"
 
 int putc(int c, FILE* f) {
-    if (f->lock < 0 || !__lockfile(f))
+    if (atomic_load(&f->lock) < 0 || !__lockfile(f))
         return putc_unlocked(c, f);
     c = putc_unlocked(c, f);
     __unlockfile(f);