arch/x86: Indirect Branch Tracking support

Indirect Branch Tracking (IBT) is one of the capabilities provided by
Intel Control-flow Enforcement Technology (CET), aimed at defending
against Jump/Call Oriented Programming.

This patch enables it for x86 (32-bit, 64-bit support coming in future
patches):

    - Add relevant Kconfigs (everything is behind X86_CET);
    - Code to enable it;
    - Enable compiler flags to enable it;
    - Add `endbr32` instructions to asm code, where needed.

Points in the code where an indirect branch is expected to land need
special instructions that tell the CPU they are valid indirect branch
targets. Those are added by the compiler, so toolchain support is
necessary. Note that any code added to the final ELF also need those
markers, such as libc or libgcc.

Finally, tests added to ensure IBT behaves sanely.

Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>