Use '__global_pointer$' special linker symbol to set gp,

because its value can be different from __BSS_END__ - 0x800.

Details from binutils-2.40/ld/emulparams/elf32lriscv-defs.sh
// We must cover as much of sdata as possible if it exists.  If sdata+bss is
// smaller than 0x1000 then we should start from bss end to cover as much of
// the program as possible.  But we can't allow gp to cover any of rodata, as
// the address of variables in rodata may change during relaxation, so we start
// from data in that case.
OTHER_END_SYMBOLS="${CREATE_SHLIB-__BSS_END__ = .;
    __global_pointer$ = MIN(__SDATA_BEGIN__ + 0x800,
                            MAX(__DATA_BEGIN__ + 0x800, __BSS_END__ - 0x800));}"

rtl/linux/riscv64/si_c.inc

@@ -16,7 +16,7 @@
  ******************************************************************************}
 
 var
-  BSS_START: record end; external name '__bss_start';
+  global_pointer: record end; external name '__global_pointer$';
 
 procedure ini_dummy;
   begin
@@ -48,7 +48,7 @@ procedure _FPC_proc_start; assembler; nostackframe; public name '_start';
     .option push
     .option norelax
 .L1:
-    auipc gp, %pcrel_hi(BSS_START+0x800)
+    auipc gp, %pcrel_hi(global_pointer)
     addi  gp, gp, %pcrel_lo(.L1)
     .option pop
 
@@ -84,7 +84,7 @@ procedure _FPC_proc_haltproc(e:longint); cdecl; public name '_haltproc';
      .option push
      .option norelax
    .L1:
-     auipc gp, %pcrel_hi(BSS_START+0x800)
+     auipc gp, %pcrel_hi(global_pointer)
      addi  gp, gp, %pcrel_lo(.L1)
      .option pop
      jalr x0, x1

rtl/linux/riscv64/si_g.inc

@@ -18,7 +18,7 @@
 {$linklib gcc}
 
 var
-  BSS_START: record end; external name '__bss_start';
+  global_pointer: record end; external name '__global_pointer$';
   _etext: pointer; external name '_etext';
 
 procedure ini_dummy;
@@ -58,7 +58,7 @@ procedure _FPC_proc_start; assembler; nostackframe; public name '_start';
     .option push
     .option norelax
 .L1:
-    auipc gp, %pcrel_hi(BSS_START+0x800)
+    auipc gp, %pcrel_hi(global_pointer)
     addi  gp, gp, %pcrel_lo(.L1)
     .option pop
 
@@ -94,7 +94,7 @@ procedure _FPC_proc_haltproc(e:longint); cdecl; public name '_haltproc';
      .option push
      .option norelax
    .L1:
-     auipc gp, %pcrel_hi(BSS_START+0x800)
+     auipc gp, %pcrel_hi(global_pointer)
      addi  gp, gp, %pcrel_lo(.L1)
      .option pop
      jalr x0, x1

rtl/linux/riscv64/si_prc.inc

@@ -19,7 +19,7 @@ var
   dlexitproc : pointer;
 
 var
-  BSS_START: record end; external name '__bss_start';
+  global_pointer: record end; external name '__global_pointer$';
   STACK_PTR: record end; external name '__stkptr';
 
 procedure _FPC_rv_enter(sp: pptruint);
@@ -42,7 +42,7 @@ procedure _FPC_proc_start; assembler; nostackframe; public name '_start';
     .option push
     .option norelax
 .L1:
-    auipc gp, %pcrel_hi(BSS_START+0x800)
+    auipc gp, %pcrel_hi(global_pointer)
     addi  gp, gp, %pcrel_lo(.L1)
     .option pop