RISC-V: Initialize the jvt CSR

Set symbol '__jvt_base$' as weak. So if the symbol is not set in the
linker script, the address would be 0. We initialize jvt CSR only if
the address is not 0.

Also use csr number directly instead of using symbolic name to prevent the
backward incompatible issue.

psabi reference:
2d770815dc/riscv-elf.adoc (table-jump-relaxation)

libgloss/riscv/crt0.S

@@ -26,6 +26,17 @@ _start:
   addi  gp, gp, %pcrel_lo(1b)
 .option pop
 
+  /* Initialize jvt CSR (reg addr: 0x0017) */
+  .weak __jvt_base$
+  lla a0, __jvt_base$
+  beqz a0, .Ljvt_init_end
+.option push
+.option norelax
+.option arch, +zicsr
+  csrw 0x17, a0
+.option pop
+.Ljvt_init_end:
+
   # Clear the bss segment
   la      a0, __bss_start
   la      a2, _end