simpleperf: support [vdso].

simpleperf: support [vdso].

Before this CL, there is no way to parse symbols from [vdso] or unwind
through it. In this CL, simpleperf dumps [vdso] segment in its own memory space
to local file system, so it can be used for getting symbols or unwinding.
It takes care that vdso files for 32bit version and 64bit version are not
misused.

Bug: None.
Test: run simpleperf_unit_test.
Test: run simpleperf on processes using vdso.

Change-Id: I9233daf1d07df262a4a0fcdeadd3e544f3ccc906