gdb/testsuite: Fix set_unbuffered_mode.o handling in parallel mode

Cygwin/MinGW testing links in a set_unbuffered_mode.o object to all
test programs.  When running the testsuite in parallel mode, on
Cygwin, I noticed errors like:

  ERROR: remote_download to host of ..../build/set_unbuffered_mode.o to ..../build/set_unbuffered_mode_saved.o: cp: cannot open '..../build/set_unbuffered_mode.o' for reading: No such file or directory
...
  ERROR: remote_download to host of ..../build/set_unbuffered_mode.o to ..../build/set_unbuffered_mode_saved.o: cp: cannot stat '..../build/set_unbuffered_mode.o': No such file or directory
...
  ERROR: remote_download to host of ..../build/set_unbuffered_mode.o to ..../build/set_unbuffered_mode_saved.o: cp: skipping file '..../build/set_unbuffered_mode.o', as it was replaced while being copied

(Absolute paths elided above.)

The problem is that gdb_compile's unbuffered_mode_obj cache isn't
parallel safe.  This is fixed in this commit.

Reviewed-by: Kevin Buettner <kevinb@redhat.com>
Change-Id: I67a289473c14ce0603d4b0beb755b124588f18d2