cfi: add __cficanonical

With CONFIG_CFI_CLANG, the compiler replaces a function address taken
in C code with the address of a local jump table entry, which passes
runtime indirect call checks. However, the compiler won't replace
addresses taken in assembly code, which will result in a CFI failure
if we later jump to such an address in instrumented C code. The code
generated for the non-canonical jump table looks this:

  <noncanonical.cfi_jt>: /* In C, &noncanonical points here */
	jmp noncanonical
  ...
  <noncanonical>:        /* function body */
	...

This change adds the __cficanonical attribute, which tells the
compiler to use a canonical jump table for the function instead. This
means the compiler will rename the actual function to <function>.cfi
and points the original symbol to the jump table entry instead:

  <canonical>:           /* jump table entry */
	jmp canonical.cfi
  ...
  <canonical.cfi>:       /* function body */
	...

As a result, the address taken in assembly, or other non-instrumented
code always points to the jump table and therefore, can be used for
indirect calls in instrumented code without tripping CFI checks.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>   # pci.h
Tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210408182843.1754385-3-samitolvanen@google.com