cmd/compile: use runtime.zeroVal for small zero-valued readonly static vars

[josharian]

Split out from #23929. No reason to have lots of different zero-valued readonly symbols when we can re-use runtime.zeroVal.

[mvdan]

Is this for performance, or to reduce binary size? Or perhaps both?

[martisch]

Likely both.

[josharian]

Performance: lower cache impact of reading zero values.

Binary size: fewer symbols.

Toolspeed: less to link.

I’d expect very minor improvements on all three.

[gopherbot]

Change https://golang.org/cl/186477 mentions this issue: cmd/compile: optimize comparisons with zero values

[agnivade]

This was my first serious foray into the front-end. I wanted to take this opportunity and thank @martisch for guiding me on this, and his patience for bearing with my questions while stumbling around the front-end code. Thanks Martin !