Properly declare and use risc-v global pointer

In our current mockturtle firmware, we declare the global pointer position like this, just before the .sdata section:

/* Have _gp point to middle of sdata/sbss to maximize displacement range */                                                                                                                                                                                                                                                 
. = ALIGN(16);                                                                                                                                                                                                                                                                                                              
_gp = . + 0x800;

And then we initialise the register like this:

la gp, _gp                 /* Initialize global pointer */

There are several issues with the above approach:

1. The most important, it seems that "recent" versions of the toolchain (I tested with gcc 7.3 and binutils 2.30) expect the label __global_pointer$, not _gp. As long as we use the old name, the toolchain does not recognise this as the "global pointer", which is a software convention and will not try to perform any gp-based relaxations. Here's the relevant output from ld -verbose that shows what is actually expected:

.sdata :
  {
    __global_pointer$ = . + 0x800;
    *(.srodata.cst16) *(.srodata.cst8) *(.srodata.cst4) *(.srodata.cst2) *(.srodata .srodata.*)
    *(.sdata .sdata.* .gnu.linkonce.s.*)
  }

2. Also, note in the above example that the global pointer location is defined inside .sdata. We do it before and we risk again having the wrong address, as in !3 (merged)
3. Finally, it is strongly recommended to "protect" the initialisation of the global pointer register. See slide 7 of the following presentation: https://riscv.org/wp-content/uploads/2019/03/11.15-Shiva-Chen-Compiler-Support-For-Linker-Relaxation-in-RISC-V-2019-03-13.pdf