abseil-cpp: Enable LTO/GC and prune RDEPENDS

Enable -ffunction-sections, -fdata-sections, -Wl,--gc-sections, and
CMake Interprocedural Optimization (LTO) so the linker strips dead code
across translation units in Abseil shared libraries.

Override split_dynamic_packages() without appending all 91 libabsl-*
subpackages to RDEPENDS:${PN}. BitBake's automatic shlibdeps scanner
(do_package_shlibs) inspects ELF DT_NEEDED entries of all target
binaries and installs only the required libabsl-* packages into the
rootfs. This drops 17-18 unreferenced debug/test/profiler shared
libraries (~1.16 MiB raw on aarch64, ~191 KiB raw on arm7a) while
keeping all 91 libraries staged in recipe-sysroot for build-time
linking.

Tested:
Built obmc-phosphor-image for aarch64 and armv7a.
Verified via readelf -d across all rootfs
ELF files (702 on aarch64, 739 on arm7a) that 100% of DT_NEEDED
dependencies are satisfied with 0 missing libraries.
- platform5: http://fusion2/66072b58-7553-32b4-a2e0-260368a1caae
- platform15: http://fusion2/181bde89-1841-314e-935f-519a2ed964ef
- platform18: http://fusion2/b1878938-0d91-3f37-bc2a-7d74e2c8bf62
- platform17: http://fusion2/cf8e0318-483a-3ba7-ad5f-9448eef74430
- platform11+ext: http://fusion2/f829b588-32c1-37dd-903b-90b52416d63c
- platform11: http://fusion2/fb81a76b-f409-37cf-9bae-9a6ea8fd8f3a
- platform25: http://fusion2/da8155f3-fcc6-3d8b-9857-8ecd5f6d16d8
Google-Bug-Id: 481049523
Change-Id: I5517cbf9998a4e5cda4d64a3f12e5f57a9968302
Signed-off-by: Willy Tu <wltu@google.com>

recipes-devtools/abseil-cpp/abseil-cpp_%.bbappend

diff --git a/recipes-devtools/abseil-cpp/abseil-cpp_%.bbappend b/recipes-devtools/abseil-cpp/abseil-cpp_%.bbappend
index b66a7d7..68393ab 100644
--- a/recipes-devtools/abseil-cpp/abseil-cpp_%.bbappend
+++ b/recipes-devtools/abseil-cpp/abseil-cpp_%.bbappend
@@ -3,3 +3,31 @@
 FILESEXTRAPATHS:prepend:gbmc := "${THISDIR}/${PN}:"
 
 SRC_URI:append:gbmc = " file://0001-Set-NDEBUG-to-true-to-avoid-DebugOnlyChecks.patch"
+
+CFLAGS += "-ffunction-sections -fdata-sections"
+CXXFLAGS += "-ffunction-sections -fdata-sections"
+LDFLAGS += "-Wl,--gc-sections"
+
+EXTRA_OECMAKE += " \
+    -DCMAKE_INTERPROCEDURAL_OPTIMIZATION=ON \
+"
+
+# Do not append all ~90 libabsl-* packages to RDEPENDS:${PN}.
+# BitBake's automatic shlibdeps (package_do_shlibs) scans DT_NEEDED entries
+# of all target binaries/libraries and installs only the exact libabsl-*
+# packages required at runtime, avoiding 16 unreferenced test/debug/profiler
+# shared libraries (~1.08 MB on aarch64).
+python split_dynamic_packages() {
+    libdir = d.getVar('libdir')
+
+    do_split_packages(
+        d,
+        root=libdir,
+        file_regex=r'^libabsl_(.*)\.so\..*$',
+        output_pattern='libabsl-%s',
+        description="abseil shared library %s",
+        prepend=True,
+        extra_depends='',
+    )
+}
+