RISC-V: Clarify the behaviors of SET/ADD/SUB relocations.

RISC-V: Clarify the behaviors of SET/ADD/SUB relocations.

We are used to generate these kinds of relocations by data directives.
Considering the following example,
.word (A + 3) - (B + 2)
The GAS will generate a pair of ADD/SUB for this,
R_RISCV_ADD, A + 1
R_RISCV_SUB, 0

The addend of R_RISCV_SUB will always be zero, and the summary of the
constants will be stored in the addend of R_RISCV_ADD/SET.  Therefore,
we can always add the addend of these data relocations when doing relocations.

But unfortunately, I had heard that if we are using .reloc to generate
the data relocations will make the relocations failed.  Refer to this,
.reloc offset, R_RISCV_ADD32, A + 3
.reloc offset, R_RISCV_SUB32, B + 2
.word 0
Then we can get the relocations as follows,
R_RISCV_ADD, A + 3
R_RISCV_SUB, B + 2
Then...  Current LD does the relocation, B - A + 3 + 2, which is wrong
obviously...

So first of all, this patch fixes the wrong relocation behavior of
R_RISCV_SUB* relocations.

Afterwards, considering the uleb128 direcitve, we will get a pair of
SET_ULEB128/SUB_ULEB128 relocations for it for now,
.uleb128 (A + 3) - (B + 2)
R_RISCV_SET_ULEB128, A + 1
R_RISCV_SUB_ULEB128, B + 1

Which looks also wrong obviously, the summary of the constants should only
be stored into the addend of SET_ULEB128, and the addend of SUB_ULEB128 should
be zero like other SUB relocations.  But the current LD will still get the right
relocation values since we only add the addend of SUB_ULEB128 by accident...
Anyway, this patch also fixes the behaviors above, to make sure that no matter
using .uleb128 or .reloc directives, we should always get the right values.

bfd/
	* elfnn-riscv.c (perform_relocation):  Clarify that SUB relocations
	should substract the addend, rather than add.
	(riscv_elf_relocate_section): Since SET_ULEB128 won't go into
	perform_relocation, we should add it's addend here in advance.
gas/
	* config/tc-riscv.c (riscv_insert_uleb128_fixes): Set the addend of
	SUB_ULEB128 to zero since it should already be added into the addend
	of SET_ULEB128.