Fix detection of cross compilation

Update cross compilation detection to not detect clang as cross compiler any more if we are compiling for host system.

Note: the problem existed because gcc and clang have different host triplets for the local machine by default:
`gcc -dumpmachine`: x86_64-linux-gnu
`clang -dumpmachine`: x86_64-pc-linux-gnu
This patch modifies the cross compilation detection so that only the architecture (x86_64) and the os (linux-gnu) are compared, because gcc has no vendor set.