NativeProcessLinux: Remove some register context boilerplate

Summary:
This patch follows the spirit of D63594, and removes some null checks
for things which should be operating invariants. Specifically
{Read,Write}[GF]PR now no longer check whether the supplied buffers are
null, because they never are. After this, the Do*** versions of these
function no longer serve any purpose and are inlined into their callers.

Other cleanups are possible here too, but I am taking this one step at a
time because this involves a lot of architecture-specific code, which I
don't have the hardware to test on (I did do a build-test though).

Reviewers: mgorny, jankratochvil, omjavaid, alexandreyy, uweigand

Subscribers: nemanjai, javed.absar, kbarton, lldb-commits

Differential Revision: https://reviews.llvm.org/D66744

llvm-svn: 370653