Sort out ambiguity of GhcDebugged

The make build system has long had a variable named GhcDebugged. The variable seems to have two interpretations depending upon who you ask:

• the testsuite driver uses it to determine whether GHC was built with -DDEBUG and consequently does assertion checking

• the rest of the build system uses it to determine whether GHC is linked against the -debug RTS.

Often these two meanings align, but they do not always. In particular, the validate way builds with -DDEBUG but not -debug.