bfd / PE/COFF: permit SEC_READONLY and consolidate

bfd / PE/COFF: permit SEC_READONLY and consolidate

Without SEC_READONLY, even the default .text section would end up with
the IMAGE_SCN_MEM_WRITE flag, which is clearly wrong. (Afaict the flag,
oddly enough, doesn't affect the final linked binary, so there likely
are more anomalies.)

However, as suggested by Richard Earnshaw, PE's requirements are really
target independent. Consolidate SEC_CODE, SEC_DATA, SEC_READONLY,
SEC_LINK_ONCE, and SEC_LINK_DUPLICATES into target-independent code. Yet
of course targets not using CREATE_*_COFF_TARGET_VEC() won't benefit.

Note that in coff-arm.c the EXTRA_S_FLAGS conditional was also stale
(leftover from EPOC removal), and hence can go away.