perf tools: use XSI-complaint version of strerror_r() instead of GNU-specific

perf tools: use XSI-complaint version of strerror_r() instead of GNU-specific

Perf uses GNU-specific version of strerror_r(). The GNU-specific strerror_r()
returns a pointer to a string containing the error message.  This may be either
a pointer to a string that the function stores in buf, or a pointer to some
(immutable) static string (in which case buf is unused).

In glibc-2.16 GNU version was marked with attribute warn_unused_result.  It
triggers few warnings in perf:

util/target.c: In function ‘perf_target__strerror’:
util/target.c:114:13: error: ignoring return value of ‘strerror_r’, declared with attribute warn_unused_result [-Werror=unused-result]
ui/browsers/hists.c: In function ‘hist_browser__dump’:
ui/browsers/hists.c:981:13: error: ignoring return value of ‘strerror_r’, declared with attribute warn_unused_result [-Werror=unused-result]

They are bugs.

Let's fix strerror_r() usage.

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>
Acked-by: Ulrich Drepper <drepper@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Ulrich Drepper <drepper@gmail.com>
Link: http://lkml.kernel.org/r/20120723210654.GA25248@shutemov.name
[ committer note: s/assert/BUG_ON/g ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>