cfi: Add type helper macros

With CONFIG_CFI_CLANG, assembly functions called indirectly
from C code must be annotated with type identifiers to pass CFI
checking.  In order to make this easier, the compiler emits a
__kcfi_typeid_<function> symbol for each address-taken function
declaration in C, which contains the expected type identifier that
we can refer to in assembly code.

Add a typed version of SYM_FUNC_START, which emits the type
identifier before the function. Architectures that support KCFI can
define their own __CFI_TYPE macro to override the default preamble
format.

As an example, for the x86_64 blowfish_dec_blk function, the
compiler emits the following type symbol:

$ readelf -sW vmlinux | grep __kcfi_typeid_blowfish_dec_blk
120204: 00000000ef478db5     0 NOTYPE  WEAK   DEFAULT  ABS
	__kcfi_typeid_blowfish_dec_blk

And SYM_TYPED_FUNC_START will generate the following preamble based
on the __CFI_TYPE definition for the architecture:

$ objdump -dr arch/x86/crypto/blowfish-x86_64-asm_64.o
     ...
0000000000000400 <__cfi_blowfish_dec_blk>:
     ...
     40b:       b8 00 00 00 00          mov    $0x0,%eax
                   40c: R_X86_64_32 __kcfi_typeid_blowfish_dec_blk

0000000000000410 <blowfish_dec_blk>:
     ...

Note that the address of all assembly functions annotated with
SYM_TYPED_FUNC_START must be taken in C code that's linked into the
binary or the missing __kcfi_typeid_ symbol will result in a linker
error with CONFIG_CFI_CLANG. If the code that contains the indirect
call is not always compiled in, __ADDRESSABLE(functionname) can be
used to ensure that the __kcfi_typeid_ symbol is emitted.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Tested-by: Kees Cook <keescook@chromium.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220908215504.3686827-7-samitolvanen@google.com
