Always rely on CFI unwind info for linux vdso

Summary:
The vdso is full of hand-written assembly which the instruction emulator has a hard time
understanding. Luckily, the kernel already provides us with correct unwind info for them. So
let's use it.

This fixes (at least) the AssertingInferiorTestCase.test_inferior_asserting_disassemble test on
android N i386.

Reviewers: tberghammer

Subscribers: tberghammer, danalbert, lldb-commits

Differential Revision: https://reviews.llvm.org/D24079

llvm-svn: 280264