Let cpp.linkerName default to cpp.compilerName for gcc-like compilers.

The hardcoded default value might not be the right one in all cases
(e.g. mingw). This solution is better than setting the linker name
in detect-toolchains, because it will also work with existing profiles.

Change-Id: Ib658090218b3191a6f554ea397c689a94c804c61
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>