Use process_vm_readv to read memory.

Modify BacktracePtrace::Read to use the process_vm_readv syscall
instead of doing multiple ptrace calls.

Test: Ran the backtrace unit tests on host and on x86_64 emulator.

Change-Id: I24c2787f6713d8eb44312f3aa751d1f6d9d34719
1 file changed