Add note to R_RISCV_PCREL_LO12_[IS]

-  GNU toolchain might use with addend, but it's deprecated.

Author: kito-cheng

riscv-elf.md

@@ -548,8 +548,8 @@ Enum | ELF Reloc Type        | Description                     | Field       | C
 21   | R_RISCV_TLS_GOT_HI20  | PC-relative TLS IE GOT offset   | _U-Type_    |             | Macro `la.tls.ie`
 22   | R_RISCV_TLS_GD_HI20   | PC-relative TLS GD reference    | _U-Type_    |             | Macro `la.tls.gd`
 23   | R_RISCV_PCREL_HI20    | PC-relative reference           | _U-Type_    | S + A - P   | `%pcrel_hi(symbol)`
-24   | R_RISCV_PCREL_LO12_I  | PC-relative reference           | _I-type_    | S - P       | `%pcrel_lo(address of %pcrel_hi)`, the addend must be 0
-25   | R_RISCV_PCREL_LO12_S  | PC-relative reference           | _S-Type_    | S - P       | `%pcrel_lo(address of %pcrel_hi)`, the addend must be 0
+24   | R_RISCV_PCREL_LO12_I  | PC-relative reference           | _I-type_    | S - P       | `%pcrel_lo(address of %pcrel_hi)`, the addend must be 0 (*note 1)
+25   | R_RISCV_PCREL_LO12_S  | PC-relative reference           | _S-Type_    | S - P       | `%pcrel_lo(address of %pcrel_hi)`, the addend must be 0 (*note 1)
 26   | R_RISCV_HI20          | Absolute address                | _U-Type_    | S + A       | `%hi(symbol)`
 27   | R_RISCV_LO12_I        | Absolute address                | _I-Type_    | S + A       | `%lo(symbol)`
 28   | R_RISCV_LO12_S        | Absolute address                | _S-Type_    | S + A       | `%lo(symbol)`
@@ -593,6 +593,13 @@ This section and later ones contain fragments written in assembler. The precise
 assembler syntax, including that of the relocations, is described in the
 [RISC-V Assembly Programmer's Manual](https://github.com/riscv/riscv-asm-manual).
 
+```
+NOTE:
+
+1: GNU Toolchain support `R_RISCV_PCREL_LO12_I` and `R_RISCV_PCREL_LO12_S`
+   with non-zero addend, but it's deprecated, and will be obsoleted in future.
+```
+
 ### Calculation Symbols
 
 The following table provides details on the variables used in relocation