s390/bug: Prevent tail-call optimization

For the exception based __WARN_trap() implementation it is technically not
necessary to prevent tail-call optimization, however it may be confusing to
see warning messages like:

WARNING: arch/s390/kernel/setup.c:1017 at foobar+0x2c/0x50, CPU#0: swapper/0/0

together with a disassembly of a different function caused by tail-call
optimization for the __WARN_trap() call. Prevent that by adding an empty
asm statement. This generates slightly worse code, but should hopefully
avoid confusion.

With this the output looks like:

WARNING: arch/s390/kernel/setup.c:1017 at foobar+0x2c/0x50, CPU#0: swapper/0/0
...
Krnl PSW : 0704c00180000000 000003ffe0119788 (foobar+0x38/0x50)
...
Krnl Code: 000003ffe0119776: e3e0f0980024        stg     %r14,152(%r15)
           000003ffe011977c: c02000b8992a        larl    %r2,000003ffe182c9d0
          *000003ffe0119782: c0e5007270b7        brasl   %r14,000003ffe0f678f0
          >000003ffe0119788: ebeff0a00004        lmg     %r14,%r15,160(%r15)
           000003ffe011978e: 07fe                bcr     15,%r14
           000003ffe0119790: 47000700            bc      0,1792
           000003ffe0119794: 0707                bcr     0,%r7
           000003ffe0119796: 0707                bcr     0,%r7
Call Trace:
 [<000003ffe0119788>] foobar+0x38/0x50
 [<000003ffe185bc2e>] arch_cpu_finalize_init+0x26/0x60
 [<000003ffe185654c>] start_kernel+0x53c/0x5d8
 [<000003ffe010002e>] startup_continue+0x2e/0x40

A better solution would be to replace or patch the branch instruction to
__WARN_trap() with the monitor call instruction, similar to what is done
for x86 [1]. However s390 does not support static_cond_calls(). Therefore
use the simple approach for the time being.

[1] commit 860238af ("x86_64/bug: Inline the UD1")

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>