build: Add support for linking with DT_RELR

This adds the -Wl,-z,pack-relative-relocs linking options.
This reduces the size of some binaries.

This is only supported on i386, x86_64, aarch64 and loongarch64 in
binutils. This feature is not support for MIPS.

musl libc supports it since version 1.2.4 .
glibc supports it since vesion 2.36.
binutils ld supports it since version 2.38 for x86 and since version
2.43 for LoongArch.

This reduces the size of the armsr default root file system from
5,262,198 bytes to 5,200,950 bytes by 61,248 bytes.

Link: https://github.com/openwrt/openwrt/pull/20679
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

config/Config-build.in

@@ -383,6 +383,15 @@ menu "Global build settings"
 			bool "Full"
 	endchoice
 
+	config PKG_DT_RELR
+		bool "Link with relative relocations (RELR)"
+		depends on (aarch64 || i386 || loongarch64 || x86_64)
+		default y
+		help
+		  Link all applications with -Wl,-z,pack-relative-relocs.
+		  This will reduce the size of many applications.
+		  This is only supported on a limited number of architectures.
+
 	config TARGET_ROOTFS_SECURITY_LABELS
 		bool
 		select KERNEL_SQUASHFS_XATTR

include/hardening.mk

@@ -8,6 +8,7 @@ PKG_ASLR_PIE_REGULAR ?= 0
 PKG_SSP ?= 1
 PKG_FORTIFY_SOURCE ?= 1
 PKG_RELRO ?= 1
+PKG_DT_RELR ?= 1
 
 ifdef CONFIG_PKG_CHECK_FORMAT_SECURITY
   ifeq ($(strip $(PKG_CHECK_FORMAT_SECURITY)),1)
@@ -69,3 +70,10 @@ ifdef CONFIG_PKG_RELRO_FULL
   endif
 endif
 
+ifdef CONFIG_PKG_DT_RELR
+  ifeq ($(strip $(PKG_DT_RELR)),1)
+    TARGET_CFLAGS += -Wl,-z,pack-relative-relocs
+    TARGET_LDFLAGS += -zpack-relative-relocs
+  endif
+endif
+