binutils-2_28-branch: Assorted RISC-V Fixes

[Palmer Dabbelt]

Since 2.28 was released a few fixes have landed on master that I think are sane
for the 2.28 branch.  The patches have better descriptions, but they are:

 * Fixes to allow assembly of a half dozen or so compressed instructions.
 * Debug info relocation fixes.
 * Help text/documentation.

Are these OK for the 2.28 branch?

[PATCH 1/7] RISC-V: Fix [dis]assembly of srai/srli
[PATCH 2/7] RISC-V: Fix the offset of CFA relocation.
[PATCH 3/7] RISC-V: Fix DW_CFA_advance_loc relocation.
[PATCH 4/7] RISC-V: Define DWARF2_USE_FIXED_ADVANCE_PC.
[PATCH 5/7] RISC-V: Fix assembler for c.addi, rd can be x0
[PATCH 6/7] RISC-V: Fix assembler for c.li, c.andi and c.addiw
[PATCH 7/7] Sanitize RISC-V GAS help text, documentation