[NativeProcessLinux] Use fast memory reads, if the system supports it

Summary:
Memory reads using the ptrace API need to be executed on a designated thread
and in 4-byte increments. The process_vm_read syscall has no such requirements
and it is about 50 times faster. This patch makes lldb-server use the faster
API if the target kernel supports it. Kernel support for this feature is
determined at runtime. Using process_vm_writev in the same manner is more
complicated since this syscall (unlike ptrace) respects page protection settings
and so it cannot be used to set a breakpoint, since code pages are typically
read-only. However, memory writes are not currently a performance bottleneck as
they happen much more rarely.

Test Plan: all tests continue to pass

Reviewers: ovyalov, vharron

Subscribers: tberghammer, lldb-commits

Differential Revision: http://reviews.llvm.org/D10488

llvm-svn: 239924