Explain checkLinker errors a bit more.

For an end user hitting these errors, the most likely reason is that
they have mixed up linker and crtbegin_dynamic.o from different builds
in their out tree.

Test: Build a linux_bionic executable in a tree exhibiting b/157549171.
Test: m nothing
Bug: 157549171
Change-Id: I64bee0848ed61226da6654caf3bafb51cf46d923