[LLVM-520] LLD Fix movt/movw relocation overflow Now committed upstream.

[LLVM-521] Taking the address of an ifunc in AArch64 Prompted by a bug report and comment about pointer equivalence, spent some time looking into gold, lld and bfd behaviour to ensure that LLD is at least correct.

Other Some investigation into LLD non-support of common-page-size and whether this is significant for code-size. Used in response to query about whether LLD should change default value of max-page-size.

Thoughts about whether a linker must generate cantunwind .ARM.exidx sections for code sections missing a .ARM.exidx section. If C++ code with exceptions is interleaved with some assembly code without .ARM.exidx sections then the assembly code can match the address range of the C++ code that precedes it.