Fix intermittent failures of Lrs-race test case

After the following change the Lrs-race test case starts to intermittently
fails:

eac65dc Add basic support for the QNX operating system

When we include "config.h" into the "libunwind_i.h" we undefine
the HAVE___THREAD macro in a few lines below in #include "config.h"
pragma. The change eac65dc includes "config.h" into the "dwarf.h"
but forgets to undefine HAVE___THREAD. So now this macro has inconsistent
state among the code. Somewhere it is defined, somewhere not. In particular
it becomes defined in the mi/Gset_caching_policy.c and we do not replace
UNW_CACHE_PER_THREAD caching policy by the UNW_CACHE_GLOBAL.

The fix is rather dirty. It adds the code to undefine HAVE___THREAD in
the "dwarf.h" like we do that in the "libunwind_i.h". Probably the ideal
solution should fix per-thread caching implementation or turned it off
at all on platforms where it is not completely and correctly supported.
[MIPS] Mark run-ptrace-mapper and run-ptrace-misc as XFAIL on MIPS

Though PTRACE_SINGLESTEP is defined on MIPS Linux, the kernel does
not support that kind of request. The ptrace call failed and sets
errno to EIO and paused process is not resumed. In case of
run-ptrace-mapper and run-ptrace-misc this leads to hanged execution
because next call to wait4 never returns.

This change adds run-ptrace-mapper and run-ptrace-misc to the list
of 'expected failed' tests on MIPS targets.