Fix handling of DT_TEXTREL for an object with more than one read-only

Fix handling of DT_TEXTREL for an object with more than one read-only
segment.  According to gABI spec, presence of the tag indicates that
dynamic linker must be prepared to handle relocations against any
read-only segment, not only the segment which we, somewhat arbitrary,
declared the text.

For each read-only segment, add write permission before relocs are
processed, and return to the mapping mode requested by the phdr, after
relocs are done.

Reported, tested, and reviewed by:	emaste
PR:	207631
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks