gdb/linux-nat: Check whether /proc/pid/mem is writable

Probe whether /proc/pid/mem is writable, by using it to write to a GDB
variable.  This will be used in the following patch to avoid falling
back to writing to inferior memory with ptrace if /proc/pid/mem _is_
writable.

Change-Id: If87eff0b46cbe5e32a583e2977a9e17d29d0ed3e