CONFIGURE: Modify AmigaOS debug information format to plain stabs.

As the GCC manual notes, "On most systems that use stabs format, -g
enables use of extra debugging information that only GDB can use;
this extra information makes debugging work better in GDB but will
probably make other debuggers crash or refuse to read the program."

-gstabs produces stabs without GDB extensions, and thus the AmigaOS
debugger will work.
CONFIGURE: Fix AmigaOS4 builds with dynamic plugins.

These previously caused crashes due to missing symbols in the dynamic
symbol table.
AMIGAOS: Update CAMD midi to use output port environment variable.

If this is not present, it will use the default as previously.
AMIGAOS: Minor change to replace strcpy usage with safer strlcpy.