MINOR: compiler: add a new DO_NOT_FOLD() macro to prevent code folding

MINOR: compiler: add a new DO_NOT_FOLD() macro to prevent code folding

Modern compilers sometimes perform function tail merging and identical
code folding, which consist in merging identical occurrences of same
code paths, generally final ones (e.g. before a return, a jump or an
unreachable statement). In the case of ABORT_NOW(), it can happen that
the compiler merges all of them into a single one in a function,
defeating the purpose of the check which initially was to figure where
the bug occurred.

Here we're creating a DO_NO_FOLD() macro which makes use of the line
number and passes it as an integer argument to an empty asm() statement.
The effect is a code position dependency which prevents the compiler
from merging the code till that point (though it may still merge the
following code). In practice it's efficient at stopping the compilers
from merging calls to ha_crash_now(), which was the initial purpose.

It may also be used to force certain optimization constructs since it
gives more control to the developer.

(cherry picked from commit e06e8a2390c42260e2998c50887a09e1a78f7252)
Signed-off-by: Willy Tarreau <w@1wt.eu>