[Investigation] Verify if binary is built with SCS reserved register.

[appujee]

One requirement for reserving x18 as a platform register for ShadowCallStack is that all the binaries should be compiled with -ffixed-x18.

@samitolvanen mentioned that binary validator can check for usage of x18.

Maybe this is not needed but I'm creating a task in case this comes up later and we can refer back. If there are other usages of having a binary validator, maybe this could be worth having one?

An example from when a binary validator would(did?) have helped Apple when Cyclone had a bug handling zcz instructions.

From: https://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20171218/511205.html

Author: tnorthover
Date: Mon Dec 18 02:36:00 2017
New Revision: 320965

URL: http://llvm.org/viewvc/llvm-project?rev=320965&view=rev
Log:
AArch64: work around how Cyclone handles "movi.2d vD, #0".

For Cylone, the instruction "movi.2d vD, #0" is executed incorrectly in some rare
circumstances. Work around the issue conservatively by avoiding the instruction entirely.

This patch changes CodeGen so that problematic instructions are never
generated, and the AsmParser so that an equivalent instruction is used (with a
warning).

[appujee]

The object file will have a tag so this is not needed

[appujee]

References:

• https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/371/files
• Specify a platform reserved register riscv-non-isa/riscv-elf-psabi-doc#370 (comment)
• https://docs.google.com/presentation/d/13EJk2WGX9uNz0Q2UZ7oKlox9741Z_afeTTXILliM8_Y/edit#slide=id.g2264735202f_2_19