Add GP-relative relocations

Author: luismarques

riscv-elf.adoc

@@ -428,7 +428,13 @@ Description:: Additional information about the relocation
                                             <| S + A - P
 .2+| 46      .2+| RVC_LUI       .2+| Static  | _CI-Type_         .2+| High 6 bits of 18-bit absolute address
                                             <| S + A
-.2+| 47-50   .2+| *Reserved*                          .2+| -       |                   .2+| Reserved for future standard use
+.2+| 47      .2+| GPREL_LO12_I  .2+| Static  | _I-type_          .2+| Low 12 bits of a 32-bit GP-relative address, `%gprel_lo(symbol)`
+                                            <| S + A - GP
+.2+| 48      .2+| GPREL_LO12_S  .2+| Static  | _S-Type_          .2+| Low 12 bits of a 32-bit GP-relative address, `%gprel_lo(symbol)`
+                                            <| S + A - GP
+.2+| 49      .2+| GPREL_HI20    .2+| Static  | _U-Type_          .2+| High 20 bits of a 32-bit GP-relative address, `%gprel_hi(symbol)`
+                                            <| S + A - GP
+.2+| 50      .2+| *Reserved*                          .2+| -        |                  .2+| Reserved for future standard use
                                             <|
 .2+| 51      .2+| RELAX         .2+| Static  |                   .2+| Instruction can be relaxed, paired with a normal relocation at the same address
                                             <|