qemu user mode: rt signals not implemented for sparc guests

This bug has been copied automatically from: https://bugs.launchpad.net/qemu/+bug/1705118
Reported by 'Bruno Haible' on 2017-07-18 :

The documentation
<https://qemu.weilnetz.de/doc/qemu-doc.html#Features> says that
qemu in user mode supports POSIX signal handling.

Catching SIGSEGV according to POSIX, however, does not work on
  ppc, ppc64, ppc64le, s390x, sparc64.
It does work, however, on
  aarch64, alpha, arm, hppa, m68k, mips, mips64, sh4.

How to reproduce:
The attached program runs fine (exits with code 0) on
  - real hardware Linux/PowerPC64 (in 32-bit and 64-bit mode),
  - real hardware Linux/PowerPC64LE,
  - qemu-system-s390x emulated Linux/s390x,
  - real hardware Linux/SPARC64.
$ gcc -O -Wall testsigsegv.c; ./a.out; echo $?
0

For ppc:
$ powerpc-linux-gnu-gcc-5 -O -Wall -static testsigsegv.c -o testsigsegv-
ppc
$ ~/inst-qemu/2.9.0/bin/qemu-ppc testsigsegv-ppc
$ echo $?
3

For ppc64:
$ powerpc64-linux-gnu-gcc-5 -O -Wall -static testsigsegv.c -o testsigsegv-
ppc64
$ ~/inst-qemu/2.9.0/bin/qemu-ppc64 testsigsegv-ppc64
$ echo $?
3

For ppc64le:
$ powerpc64le-linux-gnu-gcc-5 -O -Wall -static testsigsegv.c -o
testsigsegv-ppc64le
$ ~/inst-qemu/2.9.0/bin/qemu-ppc64le testsigsegv-ppc64le
$ echo $?
3

For s390x:
$ s390x-linux-gnu-gcc-5 -O -Wall -static testsigsegv.c -o
testsigsegv-s390x
$ ~/inst-qemu/2.9.0/bin/qemu-s390x testsigsegv-s390x
$ echo $?
3
$ s390x-linux-gnu-gcc-5 -O -Wall -static testsigsegv.c
-DAVOID_LINUX_S390X_COMPAT -o testsigsegv-s390x-a
$ ~/inst-qemu/2.9.0/bin/qemu-s390x testsigsegv-s390x-a
$ echo $?
0
So, the test fails here because the Linux/s390x kernel omits the least
significant 12 bits of the fault address in the 'si_addr' field. But
qemu-s390x is not compatible with the Linux/s390x behaviour: it puts
the complete fault address in the 'si_addr' field.

For sparc64:
$ sparc64-linux-gnu-gcc-5 -O -Wall -static testsigsegv.c -o testsigsegv-
sparc64
$ ~/inst-qemu/2.9.0/bin/qemu-sparc64 testsigsegv-sparc64
Segmentation fault (core dumped)