[X86] Mark various pointer arguments in builtins as const

Enabling `-Wcast-qual` identified many casts in various system headers
that were dropping the `const` qualifier.  Fixing those missing
qualifiers pointed out that a few of the definitions of the builtins
did not properly identify their arguments as `const` pointers.  This
commit fixes those builtin definitions, and the system header files
so that they no longer drop the qualifier.

Differential Revision: https://reviews.llvm.org/D71718