build: disable Seastar exception hack

In [1], Seastar started to bypass a lock in libgcc's exception throwing
mechanism to allow scalability on large machines. The problem is documented
in [2] and reported as fixed.

In [3], testing results on a 2s96c192t machine are reported. The problem
appears indeed fixed with gcc 14's runtime (which we use, even though we
build with clang).

Given the new results, we can safely drop the exception scalability hack.
As [1] states that the hack causes the loss of a translation cache, we
may gain some performance this way.

With that, we disable the cache by defining some random macro.

[1] https://github.com/scylladb/seastar/464f5e3ae43b366b05573018fc46321863bf2fae
[2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71744
[3] https://github.com/scylladb/seastar/issues/2479#issuecomment-2427098413

Closes scylladb/scylladb#21217

configure.py

@@ -1662,6 +1662,9 @@ user_ldflags = forced_ldflags + ' ' + args.user_ldflags
 curdir = os.getcwd()
 user_cflags = args.user_cflags + f" -ffile-prefix-map={curdir}=."
 
+# Since gcc 13, libgcc doesn't need the exception workaround
+user_cflags += ' -DSEASTAR_NO_EXCEPTION_HACK'
+
 if args.target != '':
     user_cflags += ' -march=' + args.target