UPSTREAM: gcov: docs: add a note on GCC vs Clang differences

Document some things of note to gcov users:
1. GCC gcov and Clang llvm-cov tools are not compatible.
2. The use of GCC vs Clang is transparent at build-time.

Also adjust the documentation to account for the removal of config symbol
CONFIG_GCOV_FORMAT_AUTODETECT by commit 6a61b70b43c9 ("gcov: remove
CONFIG_GCOV_FORMAT_AUTODETECT").

Link: http://lkml.kernel.org/r/20190318025411.98014-4-trong@android.com


Signed-off-by: Tri Vo <trong@android.com>
Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Cc: Daniel Mentz <danielmentz@google.com>
Cc: Greg Hackmann <ghackmann@android.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Petri Gynther <pgynther@google.com>
Cc: Prasad Sodagudi <psodagud@quicinc.com>
Cc: Trilok Soni <tsoni@quicinc.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit aa069a23a220e9ab00d6837b76917952d0fb587e)
Bug: 132997968
Test: defconfig + CONFIG_GCOV_KERNEL records coverage

Change-Id: Id9a248d5e6d8fc643300230b2ab2ce902050a6ae