arm: Create proper .rdata sections for COFF
On COFF, the default read only data section is .rdata, not .rodata.
This requires a change to gas-preprocessor as well, to handle .rdata in the same way as .rodata (when converting for armasm) in https://lists.libav.org/pipermail/libav-devel/2019-January/086440.html
Edited by Martin Storsjö