[gdb/testsuite] Fix gdb.trace/entry-values.exp on riscv64-linux

[gdb/testsuite] Fix gdb.trace/entry-values.exp on riscv64-linux

On riscv64-linux, with test-case gdb.trace/entry-values.exp I run into:
...
(gdb) disassemble bar^M
Dump of assembler code for function bar:^M
   0x0000000000000646 <+0>:     addi    sp,sp,-48^M
   0x0000000000000648 <+2>:     sd      ra,40(sp)^M
   0x000000000000064a <+4>:     sd      s0,32(sp)^M
   0x000000000000064c <+6>:     addi    s0,sp,48^M
   0x000000000000064e <+8>:     mv      a5,a0^M
   0x0000000000000650 <+10>:    sw      a5,-36(s0)^M
   0x0000000000000654 <+14>:    li      a5,2^M
   0x0000000000000656 <+16>:    sw      a5,-20(s0)^M
   0x000000000000065a <+20>:    lw      a4,-20(s0)^M
   0x000000000000065e <+24>:    lw      a5,-36(s0)^M
   0x0000000000000662 <+28>:    mv      a1,a4^M
   0x0000000000000664 <+30>:    mv      a0,a5^M
   0x0000000000000666 <+32>:    jal     0x628 <foo>^M
   0x000000000000066a <+36>:    mv      a5,a0^M
   0x000000000000066c <+38>:    mv      a0,a5^M
   0x000000000000066e <+40>:    ld      ra,40(sp)^M
   0x0000000000000670 <+42>:    ld      s0,32(sp)^M
   0x0000000000000672 <+44>:    addi    sp,sp,48^M
   0x0000000000000674 <+46>:    ret^M
End of assembler dump.^M
(gdb) FAIL: gdb.trace/entry-values.exp: disassemble bar
FAIL: gdb.trace/entry-values.exp: find the call or branch instruction offset in bar
...

Fix this by setting call_insn to jal for riscv64.

Tested on riscv64-linux and x86_64-linux.