Prepare for LLVM 9 update

[nikic]

Main changes:

• In preparation for opaque pointer types, the byval attribute now takes a type. As such, the argument type needs to be threaded through to the function/callsite attribute application logic.
• On ARM the +fp-only-sp and +d16 features have become -fp64 and -d32. I've switched the target definitions to use the new names, but also added bidirectional emulation so either can be used on any LLVM version for backwards compatibility.
• The datalayout can now specify function pointer alignment. In particular on ARM Fi8 is specified, which means that function pointer alignment is independent of function alignment. I've added this to our datalayouts to match LLVM (which is something we check) and strip the fnptr alignment for older LLVM versions.
• The fmul/fadd reductions now always respect the accumulator (including for unordered reductions), so we should pass the identity instead of undef.

Open issues:

• https://reviews.llvm.org/D62106 causes linker errors with ld.bdf due to https://sourceware.org/bugzilla/show_bug.cgi?id=24784. To avoid this I've enabled RelaxELFRelocations, which results in a GOTPCRELX relocation for __tls_get_addr and avoids the issue. However, this is likely not acceptable because relax relocations are not supported by older linker versions. We may need an LLVM option to keep using PLT for __tls_get_addr despite RtLibUseGOT.

The corresponding llvm-project PR is rust-lang/llvm-project#19.

r? @ghost