s390: Implement _THIS_IP_ using inline asm

s390: Implement _THIS_IP_ using inline asm

Both GCC [1] and Clang [2] consider the generic version of _THIS_IP_ to
be broken:

        #define _THIS_IP_  ({ __label__ __here; __here: (unsigned long)&&__here; })

In particular, the address of a label is only expected to be used with a
computed goto.

While the generic version more or less works today, it is known to be
brittle and may break with current and future optimizations. For
example, Clang -O2 always returns 1 when this function is inlined:

        static inline unsigned long get_ip(void)
        { return ({ __label__ __here; __here: (unsigned long)&&__here; }); }

Fix it by overriding _THIS_IP_ in <asm/linkage.h> (which is included by
<linux/instruction_pointer.h>) using an architecture-specific inline asm
version. Additionally, avoiding taking the address of a label prevents
compilers from emitting spurious indirect branch targets (e.g. ENDBR or
BTI) under control-flow integrity schemes.

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120071
Link: https://github.com/llvm/llvm-project/issues/138272
Signed-off-by: Marco Elver <elver@google.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>