cmd/cgo: silence unaligned-access

Clang 14+ introduced a warning when using mixed packed and unpacked structs.
This can cause problems when taking an address of the unpacked struct, which
may end up having a different alignment than expected.

This is not a problem in cgo, which does not take pointers from the packed
struct.

Updated version of https://go.dev/cl/526915, which includes
"-Wunknown-warning-option" for compilers that do not have the specific flag.

Fixes #62480

Change-Id: I788c6604d0ed5267949f4367f148fa26d2116f51
Reviewed-on: https://go-review.googlesource.com/c/go/+/528935
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/cgo/out.go

@@ -910,6 +910,8 @@ func (p *Package) writeExports(fgo2, fm, fgcc, fgcch io.Writer) {
 	fmt.Fprintf(fgcc, "#pragma GCC diagnostic ignored \"-Wunknown-pragmas\"\n")
 	fmt.Fprintf(fgcc, "#pragma GCC diagnostic ignored \"-Wpragmas\"\n")
 	fmt.Fprintf(fgcc, "#pragma GCC diagnostic ignored \"-Waddress-of-packed-member\"\n")
+	fmt.Fprintf(fgcc, "#pragma GCC diagnostic ignored \"-Wunknown-warning-option\"\n")
+	fmt.Fprintf(fgcc, "#pragma GCC diagnostic ignored \"-Wunaligned-access\"\n")
 
 	fmt.Fprintf(fgcc, "extern void crosscall2(void (*fn)(void *), void *, int, size_t);\n")
 	fmt.Fprintf(fgcc, "extern size_t _cgo_wait_runtime_init_done(void);\n")
@@ -1522,6 +1524,8 @@ extern char* _cgo_topofstack(void);
 #pragma GCC diagnostic ignored "-Wunknown-pragmas"
 #pragma GCC diagnostic ignored "-Wpragmas"
 #pragma GCC diagnostic ignored "-Waddress-of-packed-member"
+#pragma GCC diagnostic ignored "-Wunknown-warning-option"
+#pragma GCC diagnostic ignored "-Wunaligned-access"
 
 #include <errno.h>
 #include <string.h>