Fix gdb.ada/extended-access.exp on riscv64-linux

Fix gdb.ada/extended-access.exp on riscv64-linux

On riscv64-linux, I ran into:
...
(gdb) print the_array.all^M
Cannot access memory at address 0x0^M
(gdb) FAIL: gdb.ada/extended-access.exp: print the_array.all
...

The problem is that the_array.p_array is set by a dynamic relocation:
...
0000000000002010 <array_data>:
        ...

0000000000002028 <the_array>:
        ...
                        2028: R_RISCV_RELATIVE  *ABS*+0x2010
...
which doesn't seem to get its value until we arrive in main.

Fix this by running to main before trying to print the_array.

Tested on riscv64-linux.

Approved-By: Tom Tromey <tom@tromey.com>