ACPICA: Replace fake flexible arrays with flexible array members

Functionally identical to ACPICA upstream pull request 813:
acpica/acpica#813

One-element arrays (and multi-element arrays being treated as
dynamically sized) are deprecated[1] and are being replaced with
flexible array members in support of the ongoing efforts to tighten the
FORTIFY_SOURCE routines on memcpy(), correctly instrument array indexing
with UBSAN_BOUNDS, and to globally enable -fstrict-flex-arrays=3.

Replace one-element array with flexible-array member in struct
acpi_resource_extended_irq. Replace 4-byte fixed-size array with 4-byte
padding in a union with a flexible-array member in struct
acpi_pci_routing_table.

This results in no differences in binary output.

Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Len Brown <lenb@kernel.org>
Cc: Robert Moore <robert.moore@intel.com>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: linux-acpi@vger.kernel.org
Link: https://lore.kernel.org/all/20221118181538.never.225-kees@kernel.org/
Signed-off-by: Kees Cook <keescook@chromium.org>

include/acpi/acrestyp.h

@@ -332,7 +332,7 @@ struct acpi_resource_extended_irq {
 	u8 wake_capable;
 	u8 interrupt_count;
 	struct acpi_resource_source resource_source;
-	u32 interrupts[1];
+	u32 interrupts[];
 };
 
 struct acpi_resource_generic_register {
@@ -679,7 +679,10 @@ struct acpi_pci_routing_table {
 	u32 pin;
 	u64 address;		/* here for 64-bit alignment */
 	u32 source_index;
-	char source[4];		/* pad to 64 bits so sizeof() works in all cases */
+	union {
+		char pad[4];	/* pad to 64 bits so sizeof() works in all cases */
+		DECLARE_FLEX_ARRAY(char, source);
+	};
 };
 
 #endif				/* __ACRESTYP_H__ */