Implement uaccess validation with objtool

[chleroy]

On x86 objtool can perform uaccess validation, that is check that no function is called with user access is enabled after user_access_begin() and before user_access_end()

First tentative at https://patchwork.ozlabs.org/project/linuxppc-dev/list/?state=*&series=363369

The challenge is to implement switch/case code walk, the way it is implemented for x86 is too weak and doesn't fit the code GCC generates for powerpc because powerpc RISC assembly provides much more flexibility to GCC than on x86.

objtool switch/case analysis needs to be re-written in a more reliable way.