RISC-V Cleanups and ABI Fixes for 4.15-rc2

This tag contains a handful of small cleanups that are a result of
feedback that didn't make it into our original patch set, either because
the feedback hadn't been given yet, I missed the original emails, or
we weren't ready to submit the changes yet.

I've been maintaining the various cleanup patch sets I have as their own
branches, which I then merged together and signed.  Each merge commit
has a short summary of the changes, and each branch is based on your
latest tag (4.15-rc1, in this case).  If this isn't the right way to do
this then feel free to suggest something else, but it seems sane to me.

Here's a short summary of the changes, roughly in order of how
interesting they are.

* libgcc.h has been moved from include/lib, where it's the only member,
  to include/linux.  This is meant to avoid tab completion conflicts.
* VDSO entries for clock_get/gettimeofday/getcpu have been added.  These
  are simple syscalls now, but we want to let glibc use them from the
  start so we can make them faster later.
* A VDSO entry for instruction cache flushing has been added so
  userspace can flush the instruction cache.
* The VDSO symbol versions for __vdso_cmpxchg{32,64} have been removed,
  as those VDSO entries don't actually exist.
* __io_writes has been corrected to respect the given type.
* A new READ_ONCE in arch_spin_is_locked().
* __test_and_op_bit_ord() is now actually ordered.
* Various small fixes throughout the tree to enable allmodconfig to
  build cleanly.
* Removal of some dead code in our atomic support headers.
* Improvements to various comments in our atomic support headers.
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