Delay checking whether /proc/pid/mem is writable (PR gdb/29907)

As of 1bcb0708 ("gdb/linux-nat: Check whether /proc/pid/mem is
writable"), GDB checks if /proc/pid/mem is writable.  This is done
early at GDB startup, in order to get a consistent warning, instead of
a warning that depends on whenever GDB writes to inferior memory.

PR gdb/29907 points out that some build systems (like QEMU's,
apparently) may call 'gdb --version' to check GDB's presence & its
version on the system, and that Gentoo's build process has sandboxing
which blocks the /proc/pid/mem access and thus GDB warns, which
results in build fails.

To help with that, this patch delays the /proc/pid/mem check until we
start or attach to an inferior.  Ends up potentially emiting a warning
close where we already emit other ptrace- and /proc- related warnings,
which just Feels Right.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29907
Change-Id: I5537653ecfbbe76a04ab035e40e59d09b4980763