tracing: Add regex for weak functions in recordmcount.pl

Add a variable to contain the regex needed to find weak functions
in the 'nm' output. This will allow other archs to easily override it.

Also rename the regex variable $nm_regex to $local_regex to be more
descriptive.

Signed-off-by: Li Hong <lihong.hi@gmail.com>
LKML-Reference: <20091028050619.GF30758@uhli>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>