FWIW: MIPS can't represent a PC-relative offset in relocatable objects
either, but elf-eh-frame.c tries pretty hard to convert absolute addresses
into PC-relative ones. This is enough for --eh-frame-hdr to work correctly
(AFAIK) on mips*-linux-gnu.
I'd be interested to know why that code isn't kicking in for Nios II.
Maybe a simple extension would fix it, or maybe not :-) The GCC backend
needs a definition like this (from MIPS) for the conversion to work:
[snip]
#define ASM_PREFERRED_EH_DATA_FORMAT(CODE,GLOBAL) \
(((GLOBAL) ? DW_EH_PE_indirect : 0) | DW_EH_PE_absptr)
As the comment says, having PC-relative encodings is useful not only
for the .eh_frame_hdr optimisation (which can be pretty important on its own),
but because it allows .eh_frame to go in the text rather than data segment.