Fix bug 1768 - cache results from gcc_as_cc.pl

This commit moves the output of gcc_as_cc.pl from a variable to a makefile fragment: gcc_as_cc.mak. It is generated by the same perl script, which has been modified to save its parameters in comment lines in the file. If the command line parameters and the comments agree, the long slow work of generating the file will not be repeated upon reconfigure.

The new makefile fragment is only created if needed and is only included by upcc.mak.in if needed. If it does exist, the file is installed and is removed by the distclean and uninstall targets. If the user happens to delete the file from their build directory, there is a make rule to rebuild it.

Additionally, the gcc_as_cc.pl script has bee changed to use the conventional "conftest" prefix for its temporaries.