meson: build checksums with extra optimization flags.

Use -funroll-loops and -ftree-vectorize when building checksum.c to
match what autoconf does.

Missed backport of 9af672bcb2, noticed by Nathan Bossart.

Reported-by: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://postgr.es/m/a81f2f7ef34afc24a89c613671ea017e3651329c.camel@j-davis.com
Reviewed-by: Andres Freund <andres@anarazel.de>
Backpatch-through: 16

src/backend/storage/page/meson.build

@@ -1,7 +1,15 @@
 # Copyright (c) 2022-2025, PostgreSQL Global Development Group
 
+checksum_backend_lib = static_library('checksum_backend_lib',
+  'checksum.c',
+  dependencies: backend_build_deps,
+  kwargs: internal_lib_args,
+  c_args: vectorize_cflags + unroll_loops_cflags,
+)
+
+backend_link_with += checksum_backend_lib
+
 backend_sources += files(
   'bufpage.c',
-  'checksum.c',
   'itemptr.c',
 )