[gdb/testsuite] Use require in gdb.dwarf2/implptr.exp

In test-case gdb.dwarf2/implptr.exp I noticed:
...
} elseif {![is_x86_like_target]} {
    # This test can only be run on x86 targets.
    unsupported "needs x86-like target"
    return 0
}
...

Use instead "require is_x86_like_target".

Tested on x86_64-linux.