Build gdbserver for multiple targets, all at once.

The set of targets is presently hard coded.  See Makefile.in.

Usage:

Put the compilers for all targets in the PATH.  E.g.,
aarch64-linux-gnu-gcc, i686-pc-linux-gnu-gcc, etc.  Then:

 $ mkdir build
 $ cd build
 $ /path/to/configure --with-srcdir=/path/to/src/gdb/gdbserver
 ...
 $ make -j8