scripts/sorttable: Get start/stop_mcount_loc from ELF file directly

scripts/sorttable: Get start/stop_mcount_loc from ELF file directly

The get_mcount_loc() does a cheesy trick to find the start_mcount_loc and
stop_mcount_loc values. That trick is:

 file_start = popen(" grep start_mcount System.map | awk '{print $1}' ", "r");

and

 file_stop = popen(" grep stop_mcount System.map | awk '{print $1}' ", "r");

Those values are stored in the Elf symbol table. Use that to capture those
values. Using the symbol table is more efficient and more robust. The
above could fail if another variable had "start_mcount" or "stop_mcount"
as part of its name.

Cc: bpf <bpf@vger.kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nicolas Schier <nicolas@fjasle.eu>
Cc: Zheng Yejian <zhengyejian1@huawei.com>
Cc: Martin  Kelly <martin.kelly@crowdstrike.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/20250105162346.817157047@goodmis.org
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>