Makefile: print uname in install_prerequisites
Makefile: print uname in install_prerequisites

Travis is still broken with:

E: Unable to locate package gcc-aarch64-linux-gnu
E: Unable to locate package gcc-arm-linux-gnueabihf
E: Unable to locate package gcc-powerpc64le-linux-gnu

https://travis-ci.org/google/syzkaller/jobs/378034948

Let's at least see what's OS is there and what gcc packages present.

xairy/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller