Useful for core dumps. It used to not work because:

    (gdb) p !""
    evaluation of this expression requires the target program to be active
    (gdb) p 0 == ""
    evaluation of this expression requires the target program to be active

When doing out-of-source builds I often end up wanting to use gdb from
inside a build directory.

Currently if I do that the `.gdbinit` in the ruby source directory is
not read.

Attempting to manually load the `.gdbinit` from the Ruby source
directory errors because of hard coded file-paths that assume we're
running gdb from inside the source tree.

Adding the `-s` flag to the `source` command when loading relative files
signals to gdb to search in the source search path for the required
file, rather than just the current directory.

This allows me to use a custom `.gdbinit` from my build directory that
sets the correct source directory and then loads the Ruby `.gdbinit`.

```
dir ../src
source ../src/.gdbinit
```

After a0f12a0258e4020bd657ee80b7d8f22bd33ea223 NODE_GASGN and
NODE_GVAR hold same value on both nd_vid and nd_entry.
This commit stops setting value to nd_entry and makes to use only
nd_vid.

Since [Misc #19679] migrated the wiki, these links should be updated
to their new locations.

This works like:

```
(gdb) cfp
CFP (count=3, addr=0x7ffff73fef50):
$1 = {pc = 0x555556bf7818, sp = 0x7ffff72ff078, iseq = 0x7ffff2603270, self = 140737344619296, ep = 0x7ffff72ff058, block_code = 0x0,
  __bp__ = 0x7ffff72ff060, jit_return = 0x555558c2b000}

Stack (size=3):
[0] FIXNUM: 1
[1] T_STRING: "" bytesize:0 (embed) encoding:1 coderange:7bit $2 = (struct RString *) 0x7ffff249ea80
[2] [PROMOTED] T_OBJECT: $3 = {flags = 21474844769, klass = 140737344040416}
$4 = {0x24, 0x24, 0x24}

(gdb) cfp + 1
CFP (count=3, addr=0x7ffff73fef90):
$5 = {pc = 0x5555567a78f8, sp = 0x7ffff72ff040, iseq = 0x7ffff26032d0, self = 140737344619296, ep = 0x7ffff72ff038, block_code = 0x0,
  __bp__ = 0x7ffff72ff040, jit_return = 0x555558c2b000}

Stack (size=0):
```

It was crashing with:
No symbol "ROBJECT_EMBED_LEN_MAX" in current context.