arch: fix asm-offsets.c building with -Wmissing-prototypes

When -Wmissing-prototypes is enabled, the some asm-offsets.c files fail
to build, even when this warning is disabled in the Makefile for normal
files:

arch/sparc/kernel/asm-offsets.c:22:5: error: no previous prototype for 'sparc32_foo' [-Werror=missing-prototypes]
arch/sparc/kernel/asm-offsets.c:48:5: error: no previous prototype for 'foo' [-Werror=missing-prototypes]

Address this by making use of the same trick as x86, marking these
functions as 'static __used' to avoid the need for a prototype
by not drop them in dead-code elimination.

Suggested-by: Masahiro Yamada <[e-mail]>
Reviewed-by: Sam Ravnborg <[e-mail]>
Link: https://lore.kernel.org/lkml/[e-mail]/
Signed-off-by: Arnd Bergmann <[e-mail]>

arch/alpha/kernel/asm-offsets.c

···
 #include <linux/kbuild.h>
 #include <asm/io.h>
 
-void foo(void)
+static void __used foo(void)
 {
 	DEFINE(TI_TASK, offsetof(struct thread_info, task));
 	DEFINE(TI_FLAGS, offsetof(struct thread_info, flags));

arch/loongarch/kernel/asm-offsets.c

···
 #include <asm/processor.h>
 #include <asm/ftrace.h>
 
-void output_ptreg_defines(void)
+static void __used output_ptreg_defines(void)
 {
 	COMMENT("LoongArch pt_regs offsets.");
 	OFFSET(PT_R0, pt_regs, regs[0]);
···
 	BLANK();
 }
 
-void output_task_defines(void)
+static void __used output_task_defines(void)
 {
 	COMMENT("LoongArch task_struct offsets.");
 	OFFSET(TASK_STATE, task_struct, __state);
···
 	BLANK();
 }
 
-void output_thread_info_defines(void)
+static void __used output_thread_info_defines(void)
 {
 	COMMENT("LoongArch thread_info offsets.");
 	OFFSET(TI_TASK, thread_info, task);
···
 	BLANK();
 }
 
-void output_thread_defines(void)
+static void __used output_thread_defines(void)
 {
 	COMMENT("LoongArch specific thread_struct offsets.");
 	OFFSET(THREAD_REG01, task_struct, thread.reg01);
···
 	BLANK();
 }
 
-void output_thread_fpu_defines(void)
+static void __used output_thread_fpu_defines(void)
 {
 	OFFSET(THREAD_FPR0, loongarch_fpu, fpr[0]);
 	OFFSET(THREAD_FPR1, loongarch_fpu, fpr[1]);
···
 	BLANK();
 }
 
-void output_thread_lbt_defines(void)
+static void __used output_thread_lbt_defines(void)
 {
 	OFFSET(THREAD_SCR0,  loongarch_lbt, scr0);
 	OFFSET(THREAD_SCR1,  loongarch_lbt, scr1);
···
 	BLANK();
 }
 
-void output_mm_defines(void)
+static void __used output_mm_defines(void)
 {
 	COMMENT("Size of struct page");
 	DEFINE(STRUCT_PAGE_SIZE, sizeof(struct page));
···
 	BLANK();
 }
 
-void output_sc_defines(void)
+static void __used output_sc_defines(void)
 {
 	COMMENT("Linux sigcontext offsets.");
 	OFFSET(SC_REGS, sigcontext, sc_regs);
···
 	BLANK();
 }
 
-void output_signal_defines(void)
+static void __used output_signal_defines(void)
 {
 	COMMENT("Linux signal numbers.");
 	DEFINE(_SIGHUP, SIGHUP);
···
 }
 
 #ifdef CONFIG_SMP
-void output_smpboot_defines(void)
+static void __used output_smpboot_defines(void)
 {
 	COMMENT("Linux smp cpu boot offsets.");
 	OFFSET(CPU_BOOT_STACK, secondary_data, stack);
···
 #endif
 
 #ifdef CONFIG_HIBERNATION
-void output_pbe_defines(void)
+static void __used output_pbe_defines(void)
 {
 	COMMENT("Linux struct pbe offsets.");
 	OFFSET(PBE_ADDRESS, pbe, address);
···
 #endif
 
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
-void output_fgraph_ret_regs_defines(void)
+static void __used output_fgraph_ret_regs_defines(void)
 {
 	COMMENT("LoongArch fgraph_ret_regs offsets.");
 	OFFSET(FGRET_REGS_A0, fgraph_ret_regs, regs[0]);
···
 }
 #endif
 
-void output_kvm_defines(void)
+static void __used output_kvm_defines(void)
 {
 	COMMENT("KVM/LoongArch Specific offsets.");
 

arch/sparc/kernel/asm-offsets.c

···
 #include <asm/hibernate.h>
 
 #ifdef CONFIG_SPARC32
-int sparc32_foo(void)
+static int __used sparc32_foo(void)
 {
 	DEFINE(AOFF_thread_fork_kpsr,
 			offsetof(struct thread_struct, fork_kpsr));
 	return 0;
 }
 #else
-int sparc64_foo(void)
+static int __used sparc64_foo(void)
 {
 #ifdef CONFIG_HIBERNATION
 	BLANK();
···
 }
 #endif
 
-int foo(void)
+static int __used foo(void)
 {
 	BLANK();
 	DEFINE(AOFF_task_thread, offsetof(struct task_struct, thread));