So, I notice that all sparc (ELF) executables display a weird 64KB
	gap between the end of the .text (or .rodata) segment and the start
	of the .data segment. Below's an arbitrary example objdump.

	I'm not aware of any reason this must be so. How can I fix it?

Do not fix that.  That is part of the ELF spec.

The maximum ELF pages size for SPARC is 64KB so all
segments begin at those boundaries.  (SPARC v9 uses
a 1MB maximum ELF page size.)  This allows different
segments, which have different protections, to be
located on different pages, no matter the design
of the MMU.

If you want to get rid of that gap you can combine
the segments (what is that, the -z magic flag?

Eduardo