linux-user: SPARC "rd %tick" can be used by user application

we have the same problem decribed in 7d6b1daedd
("linux-user, ppc: mftbl can be used by user application")
for ppc in the case of sparc.

When we use an application trying to resolve a name, it hangs in

    0x00000000ff5dd40c:  rd  %tick, %o5
    0x00000000ff5dd410:  srlx  %o5, 0x20, %o4
    0x00000000ff5dd414:  btst  %o5, %g4
    0x00000000ff5dd418:  be  %icc, 0xff5dd40c

because %tick is staying at 0.

As QEMU_CLOCK_VIRTUAL is not available in linux-user mode,
simply use cpu_get_host_ticks() instead.

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20180528194812.31216-1-laurent@vivier.eu>

target/sparc/helper.c

@@ -67,7 +67,9 @@ uint64_t helper_tick_get_count(CPUSPARCState *env, void *opaque, int mem_idx)
 
     return cpu_tick_get_count(timer);
 #else
-    return 0;
+    /* In user-mode, QEMU_CLOCK_VIRTUAL doesn't exist.
+       Just pass through the host cpu clock ticks.  */
+    return cpu_get_host_ticks();
 #endif
 }