perf config: Make symbol_conf::addr2line_disable_warn configurable

perf config: Make symbol_conf::addr2line_disable_warn configurable

Make symbol_conf::addr2line_disable_warn configurable by reading
the perfconfig file.
Use section core and addr2line-disable-warn = value.
Update documentation.

Example:
 # perf config -l
 core.addr2line-timeout=5000
 core.addr2line-disable-warn=1
 #

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Reviewed-by: Ian Rogers <irogers@google.com>
Suggested-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>