link.Elf: Define __global_pointer$ for riscv32 too.

See: https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-elf.adoc

src/link/Elf.zig

@@ -3150,7 +3150,7 @@ fn addLinkerDefinedSymbols(self: *Elf) !void {
         }
     }
 
-    if (self.getTarget().cpu.arch == .riscv64 and self.isEffectivelyDynLib()) {
+    if (self.getTarget().cpu.arch.isRISCV() and self.isEffectivelyDynLib()) {
         self.global_pointer_index = try linker_defined.addGlobal("__global_pointer$", self);
     }