[linux] Use cmake to detect support process_vm_readv (bug #23918)

Summary:
Some old linux versions do not have process_vm_readv function defined. Even older versions do not
have even the __NR_process_vm_readv syscall number. We use cmake to detect these situations and
fallback appropriately: in the first case, we can issue the syscall manually, while it the latter
case, we need to drop fast memory read support completely.

Test Plan: linux test suite passes

Reviewers: ovyalov, Eugene.Zelenko

Subscribers: tberghammer, lldb-commits

Differential Revision: http://reviews.llvm.org/D10727

llvm-svn: 240927