[BUG] definitely fix regparm issues between haproxy core and ebtree

[BUG] definitely fix regparm issues between haproxy core and ebtree

It's a pain to enable regparm because ebtree is built in its corner
and does not depend on the rest of the config. This causes no problem
except that if the regparm settings are not exactly similar, then we
can get inconsistent function interfaces and crashes.

One solution realized in this patch consists in externalizing all
compiler settings and changing CONFIG_XXX_REGPARM into CONFIG_REGPARM
so that we ensure that any sub-component uses the same setting. Since
ebtree used a value here and not a boolean, haproxy's config has been
set to use a number too. Both haproxy's core and ebtree currently use
the same copy of the compiler.h file. That way we don't have any issue
anymore when one setting changes somewhere.