gdb: make timestamped_file implement write_async_safe

Trying to use "set debug linux-nat 1", I get an internal error:

    /home/smarchi/src/binutils-gdb/gdb/ui-file.h:70: internal-error: write_async_safe: write_async_safe

The problem is that timestamped_file doesn't implement write_async_safe,
which linux-nat's sigchld_handler uses.  Implement it.

Change-Id: I830981010c6119f13ae673605ed015cced0f5ee8
1 file changed