docs/linux: add references to syzkaller build instructions

In the various linux build instructions, add pointers to the main
doc with instructions for building syzkaller.

These instructions were present before commit d23e90a7b44b ("all:
switch to Go 1.12"). They were duplicated across files. Since the
commit the instructions are in one place. Make it easier to find
them from the platform-specific setup guides.

Also clarify in the x86_64 guide that using a precompiled compiler
from the supplied list is optional if the distribution provided
compiler is recent enough. Before commit d28f4ce ("Update
setup_ubuntu-host_qemu-vm_x86-64-kernel.md") the file had explicit
build instructions. Those are no longer needed.

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