ubsan: disable unsigned-overflow check for i386

Building ubsan kernels even for compile-testing introduced these
warnings in my randconfig environment:

  crypto/blake2b_generic.c:98:13: error: stack frame size of 9636 bytes in function 'blake2b_compress' [-Werror,-Wframe-larger-than=]
  static void blake2b_compress(struct blake2b_state *S,

  crypto/sha512_generic.c:151:13: error: stack frame size of 1292 bytes in function 'sha512_generic_block_fn' [-Werror,-Wframe-larger-than=]
  static void sha512_generic_block_fn(struct sha512_state *sst, u8 const *src,

  lib/crypto/curve25519-fiat32.c:312:22: error: stack frame size of 2180 bytes in function 'fe_mul_impl' [-Werror,-Wframe-larger-than=]
  static noinline void fe_mul_impl(u32 out[10], const u32 in1[10], const u32 in2[10])

  lib/crypto/curve25519-fiat32.c:444:22: error: stack frame size of 1588 bytes in function 'fe_sqr_impl' [-Werror,-Wframe-larger-than=]
  static noinline void fe_sqr_impl(u32 out[10], const u32 in1[10])

Further testing showed that this is caused by
-fsanitize=unsigned-integer-overflow, but is isolated to the 32-bit x86
architecture.

The one in blake2b immediately overflows the 8KB stack area
architectures, so better ensure this never happens by disabling the
option for 32-bit x86.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lore.kernel.org/lkml/[email protected]/
Fixes: d0a3ac5 ("ubsan: enable for all*config builds")
Signed-off-by: Arnd Bergmann <[email protected]>
Acked-by: Kees Cook <[email protected]>
Reviewed-by: Nathan Chancellor <[email protected]>
Cc: Nick Desaulniers <[email protected]>
Cc: Stephen Rothwell <[email protected]>
Cc: Marco Elver <[email protected]>
Cc: George Popescu <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

lib/Kconfig.ubsan

@@ -123,6 +123,7 @@ config UBSAN_SIGNED_OVERFLOW
 config UBSAN_UNSIGNED_OVERFLOW
 	bool "Perform checking for unsigned arithmetic overflow"
 	depends on $(cc-option,-fsanitize=unsigned-integer-overflow)
+	depends on !X86_32 # avoid excessive stack usage on x86-32/clang
 	help
 	  This option enables -fsanitize=unsigned-integer-overflow which checks
 	  for overflow of any arithmetic operations with unsigned integers. This