Fix OP_GETGV symbol encoding for $&, $\, $', $1..$9`

[hadashiA]

Summary

codegen_prism.inc mis-encodes two kinds of global-variable reads in
OP_GETGV operands:

1. Back-references ($&, $`, $', $+) all use
   MRC_SYM_2(back_ref), the presym bound to the literal "$+", so
   every back-reference collapses to "$+" in the dumped bytecode.

2. Numbered references ($1..$9) build the name in a local stack
   buffer and hand the pointer to pm_constant_pool_insert_constant,
   which stores the pointer without copying. After the codegen
   function returns the buffer is reused, so the dumped symbol bytes
   are garbage (we observed \x08G for $1).

Reproduce

/world/ =~ 'hello world'
p $`, $&, $+, $1

$ mrbc -v repro.rb                                                  
... GETGV  R2  $+    # was $` in source
... GETGV  R2  $+    # was $& in source
... GETGV  R2  $+
... GETGV  R2  G     # garbage; was $1 in source                                                                                                                                                                            

Fix

• Back-reference: use cast->name (the Prism constant_id for the actual
  token bytes — already in the pool).
• Numbered reference: mrc_malloc a heap copy and hand ownership to the
  pool via pm_constant_pool_insert_owned.