compiler, libgo: don't pad sparc64-linux epollevent

Change the compiler to not add zero padding because of zero-sized
fields named "_", since those can't be referenced anyhow.

Change the sparc-linux64 epollevent struct to name the alignment
field "_", to avoid zero padding.

Fixes PR go/103847

	PR go/103847
	* godump.c (go_force_record_alignment): Name the alignment
	field "_".

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/374914