cmd/link,cmd/internal: add R_GOT_PCREL_ITYPE_RELOC for riscv64
This CL adds new relocation type for riscv64: R_GOT_PCREL_ITYPE_RELOC
which generate an AUIPC + I-type pair with relocation type of GOT_HI20
and PCREL_LO12_I.
According to RISCV elf psabi doc, medium position independent code
model, the GNU as example is:
```
# Calculate address of non-local symbol
.Ltmp3: aupipc a0, %got_pcrel_hi(symbol)
ld a0, %pcrel_lo(.Ltmp3)(a0)
```
Change-Id: I719dd05e009ca2d9291f0689b346c059f9c56918
Reviewed-on: https://go-review.googlesource.com/c/go/+/612635 Reviewed-by: Dmitri Shuralyov <dmitshur@google.com> Reviewed-by: Joel Sing <joel@sing.id.au> Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>