RISC-V LLVM sync-up call 30 Apr 2020

For background on these calls, see

Reminder: the purpose is to co-ordinate between active contributors.
If you have support questions etc then it's best to post to llvm-dev.

We have a call each Thursday at 4pm BST, via

I've created a shared calendar which may help in keeping track, which
is accessible at:
  * <https://calendar.google.com/calendar/b/1?cid=bG93cmlzYy5vcmdfMG41cGtlc2ZqY25wMGJoNWhwczFwMGJkODBAZ3JvdXAuY2FsZW5kYXIuZ29vZ2xlLmNvbQ&gt;
  * <https://calendar.google.com/calendar/ical/lowrisc.org_0n5pkesfjcnp0bh5hps1p0bd80%40group.calendar.google.com/public/basic.ics&gt;

Issues to discuss today include the following:
* mcpu/march ⚙ D71124 [RISCV] support clang driver to select cpu
* RISC-V datalayout and aligned access support
45708 – RISC-V DataLayout is wrong which causes many optimizations to not work on RISC-V
* Differential testing work and resulting / WIP patches
* [If time, and right people present] Linker relaxation,
⚙ D79105 [LLD][ELF][RISCV] Linker relaxation support for R_RISCV_CALL
* No other topics were submitted, as always, please do submit things
you'd like to discuss