x86, lto: Enable Clang LTO for 32-bit as well

Commit b33fff07 ("x86, build: allow LTO to be selected") enabled
support for LTO for x86_64 but 32-bit works fine as well.

I tested the following config combinations:

* i386_defconfig + CONFIG_LTO_CLANG_FULL=y

* i386_defconfig + CONFIG_LTO_CLANG_THIN=y

* ARCH=i386 allmodconfig + CONFIG_LTO_CLANG_THIN=y

with LLVM 11.1.0, 12.0.0, and 13.0.0 from git without any build
failures. The defconfigs boot in QEMU with no new warnings.

Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210429232611.3966964-1-nathan@kernel.org