[arch][riscv] set up the global pointer register

The linker will automatically relax any dereferences and calculations
for symbols located near the symbol __global_pointer$.

arch/riscv/linker.ld

@@ -67,6 +67,7 @@ SECTIONS
         /* in one segment binaries, the rom data address is on top of the ram data address */
         __data_start = .;
         *(.data .data.* .gnu.linkonce.d.*)
+        PROVIDE( __global_pointer$ = . + (4K / 2) );
         *(.sdata .sdata.*)
         __ctor_list = .;
         KEEP(*(.ctors .init_array))

arch/riscv/start.S

@@ -44,6 +44,12 @@ _vectab:
 
 .section ".text.boot"
 FUNCTION(_start)
+.option push
+.option norelax
+    # set the global pointer
+    la      gp, __global_pointer$
+.option pop
+
     # set the default stack
     la      sp, default_stack_top