runtime/cgo: ignore -Watomic-alignment in gcc_libinit.c

When cross-compiling a cgo program with CC=clang for Linux/ARMv5,
certain atomic warnings cause build errors, as cgo employs -Werror.

These warnings seem to be harmless and come from the usage of
__atomic_load_n, which is emulated due to the lack of atomic
instructions in armv5.

Fixes [reserved]

Change-Id: Ied596456e3ad64dfd23509924ecca18130b8148d

Author: mauri870

src/runtime/cgo/gcc_libinit.c

@@ -4,6 +4,11 @@
 
 //go:build unix
 
+// When cross-compiling to linux/armv5, atomics are emulated and cause a
+// compiler warning. This results in a build failure since cgo uses
+// -Werror. See #65290.
+#pragma GCC diagnostic ignored "-Watomic-alignment"
+
 #include <pthread.h>
 #include <errno.h>
 #include <stdio.h>