Bug 1854493 - Use -O2 on Linux debug builds. r=firefox-build-system-reviewers,ahochheiden

Linux is the only platform where we have a different optimization level
between normal and debug builds. That comes from bug 1368649, but was
not the original intent: the first landing for that bug actually set the
optimization level to -O2 for all non-PGO Linux builds, but that caused
build problems on the debug builds with the compiler used at the time.

Time has passed, compiler versions have been released, and, in fact,
back then we were still compiling with gcc for the shipped builds.

It's time to go with the original intent.

Differential Revision: https://phabricator.services.mozilla.com/D188896

old-configure.in

@@ -391,11 +391,7 @@ case "$target" in
 *-*linux*)
     if test "$GNU_CC" -o "$GNU_CXX"; then
         MOZ_PGO_OPTIMIZE_FLAGS="-O3"
-        if test -n "$MOZ_DEBUG"; then
-            MOZ_OPTIMIZE_FLAGS="-Os"
-        else
-            MOZ_OPTIMIZE_FLAGS="-O2"
-        fi
+        MOZ_OPTIMIZE_FLAGS="-O2"
         if test -z "$CLANG_CC"; then
            MOZ_OPTIMIZE_FLAGS="-freorder-blocks $MOZ_OPTIMIZE_FLAGS"
         fi