Remove further reference to R_RISCV_GPREL_[IS]

[jrtc27]

We currently still say the following:

GP-Relative Relocations: If symbol is within the range of a signed 12-bit
immediate offset from __global_pointer$, then the address can be loaded with a
single instruction which has one relocation, a R_RISCV_GPREL_I or
R_RISCV_GPREL_S.

The instruction is an I-Type instruction (add immediate or load) with an
R_RISCV_GPREL_I or an S-type instruction (store) with an R_RISCV_GPREL_S
relocation. The following assembly show loading a gp-relative address:

     addi a0, gp, 0          # R_RISCV_GPREL_I (symbol)

This relies on the value of __global_pointer$ being loaded into gp (aka
x3). This can be used by linker relaxation to delete the lui instruction.

There was never any assembly syntax for that and the relocations have since been removed due to being internal binutils relocations. We should remove this section.

[kito-cheng]

I believe this issue is resolved by #199 :)

[jrtc27]

No, that's why this issue says "further". I missed this in my original PR.

[kito-cheng]

Oh, I got your point, I guess I can working on writing __global_pointer$ in next few days and remove those sentence about R_RISCV_GPREL_[IS], or are you already working on that?

[jrtc27]

We already have https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-elf.md#calculation-symbols, I think the quoted section just needs deleting.