kstack_erase: Add -mgeneral-regs-only to silence Clang warnings

Once CONFIG_KSTACK_ERASE is enabled with Clang on i386, the build warns:

  kernel/kstack_erase.c:168:2: warning: function with attribute 'no_caller_saved_registers' should only call a function with attribute 'no_caller_saved_registers' or be compiled with '-mgeneral-regs-only' [-Wexcessive-regsave]

Add -mgeneral-regs-only for the kstack_erase handler, to make Clang feel
better (it is effectively a no-op flag for the kernel). No binary
changes encountered.

Build & boot tested with Clang 21 on x86_64, and i386.
Build tested with GCC 14.2.0 on x86_64, i386, arm64, and arm.

Reported-by: Nathan Chancellor <[email protected]>
Closes: https://lore.kernel.org/all/20250726004313.GA3650901@ax162
Tested-by: Nathan Chancellor <[email protected]>
Signed-off-by: Kees Cook <[email protected]>

Author: kees

kernel/Makefile

@@ -140,6 +140,7 @@ obj-$(CONFIG_RESOURCE_KUNIT_TEST) += resource_kunit.o
 obj-$(CONFIG_SYSCTL_KUNIT_TEST) += sysctl-test.o
 
 CFLAGS_kstack_erase.o += $(DISABLE_KSTACK_ERASE)
+CFLAGS_kstack_erase.o += $(call cc-option,-mgeneral-regs-only)
 obj-$(CONFIG_KSTACK_ERASE) += kstack_erase.o
 KASAN_SANITIZE_kstack_erase.o := n
 KCSAN_SANITIZE_kstack_erase.o := n