code.
For backward compatibility, existing usages of `//go:linkname` found in
a large open-source code corpus remain supported.
Any new references to standard library internal symbols will be disallowed.

A linker command line flag `-checklinkname=0` can be used to disable
this check, for debugging and experimenting purposes.

<!-- CL 473495 -->
When building a dynamically linked ELF binary (including PIE binary), the