perf tools: Make find_vdso_map() more modular

In preparation for checking that the vectors page on the ARM
architecture, refactor the find_vdso_map() function to accept finding an
arbitrary string and create a dedicated helper function for that under
util/find-map.c and update the filename to find-map.c and all references
to it: perf-read-vdso.c and util/vdso.c.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Chris Healy <cphealy@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Kim Phillips <kim.phillips@arm.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Cc: Russell King <rmk+kernel@armlinux.org.uk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Thomas Richter <tmricht@linux.ibm.com>
Link: http://lkml.kernel.org/r/20181221034337.26663-2-f.fainelli@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>