[gdb/testsuite] Complete 64-bit port of gdb.dwarf2/dw2-skip-prologue.exp

[gdb/testsuite] Complete 64-bit port of gdb.dwarf2/dw2-skip-prologue.exp

Test-source gdb.dwarf2/dw2-skip-prologue.S was generated using 32-bit, and
subsequently ported to 64-bit using macro PTRBYTE.

When using 64-bit, there are some readelf -w warnings because the port to
64-bit is not complete.  Fix these warnings.

Likewise in gdb.dwarf/dw2-const.exp.

Approved-By: Tom Tromey <tom@tromey.com>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=34157