UML: normalize global variables exported by vmlinux.lds

Normalize global variables exported by vmlinux.lds to conform usage
guidelines from include/asm-generic/sections.h.

1) Use _text to mark the start of the kernel image including the head
text, and _stext to mark the start of the .text section.
2) Export mandatory global variables __bss_stop.
3) Adjust __init_begin and __init_end to avoid acrossing .text and
   .data sections.

Change-Id: I8e9946f8df9a66e8134c046c84c9353b124cd739
Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Git commit: a214a8c68bcdef2fb0803425f7fe36fe41030d3f
Git repo: https://github.com/torvalds/linux.git


Signed-off-by: Lingutla Chandrasekhar <clingutla@codeaurora.org>
Signed-off-by: Srinivasarao P <spathi@codeaurora.org>