[riscv] Discard ELF hash tables

The ELF hash table is generated when building a position-independent
executable even though it is not required (since we have no dynamic
linker).

Explicitly discard these unneeded sections.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/riscv/scripts/sbi.lds

@@ -117,6 +117,8 @@ SECTIONS {
 	*(.dynamic)
 	*(.dynsym)
 	*(.dynstr)
+	*(.hash)
+	*(.gnu.hash)
 	*(.einfo)
 	*(.einfo.*)
 	*(.discard)