Document existing GNU extensions

Fri Jan 1 00:00:00 GMT 2016

There are many GNU extensions in GCC, binutils and glibc.  Some of
them are

PT_GNU_EH_FRAME
STT_GNU_IFUNC
PT_GNU_STACK
PT_GNU_RELRO
SHT_GNU_INCREMENTAL_INPUTS
SHT_GNU_ATTRIBUTES
SHT_GNU_HASH
SHT_GNU_LIBLIST
SHT_GNU_verdef
SHT_GNU_verneed
SHT_GNU_versym
STB_GNU_UNIQUE
DT_GNU_PRELINKED
DT_GNU_CONFLICTSZ
DT_GNU_LIBLISTSZ
DT_GNU_HASH
DT_GNU_CONFLICT
DT_GNU_LIBLIST
DT_RELCOUNT
DT_RELACOUNT
.note.ABI-tag
.note.GNU-stack

Most of their documentations are with the source codes or spread over
multiple email messages in various mailing list archives.  There is no
central place to look them up.  I documented 2 of them:

PT_GNU_EH_FRAME
STT_GNU_IFUNC

and put them at

https://github.com/hjl-tools/linux-abi

Is there a git repo I can use on sourceware.org?

H.J.