gdb: convert linux-osdata.c from buffer to std::string

Replace the use of struct buffer in linux-osdata.c with std::string.
There is no change in the logic, so there should be no user-visible
change.

Change-Id: I27f53165d401650bbd0bebe8ed88221e25545b3f
Approved-By: Pedro Alves <pedro@palves.net>