[Tizen] Add syscall compat header for custom Linux toolchains

Inject a small compat header into the custom cross-toolchain command line
so Linux targets built with Tizen sysroots still see __NR_getrandom even
when the sysroot headers do not export it.

This keeps third_party/dart untouched and fixes builds across common Tizen
architectures:
- x86: 355
- x64: 318
- arm: 384
- arm64: 278

Author: JSUYA

engine/src/build/toolchain/custom/BUILD.gn

@@ -20,6 +20,9 @@ toolchain("custom") {
 
   target_triple_flags = "--target=${custom_target_triple}"
   sysroot_flags = "--sysroot ${custom_sysroot}"
+  syscall_compat_header =
+      rebase_path("//build/toolchain/custom/tizen_syscall_compat.h", root_build_dir)
+  syscall_compat_flags = "-include ${syscall_compat_header}"
 
   custom_lib_flags = "-L${custom_toolchain}/lib"
 
@@ -29,7 +32,7 @@ toolchain("custom") {
 
   tool("cc") {
     depfile = "{{output}}.d"
-    command = "$cc -MD -MF $depfile $target_triple_flags $sysroot_flags {{defines}} {{include_dirs}} {{cflags}} {{cflags_c}} -c {{source}} -o {{output}}"
+    command = "$cc -MD -MF $depfile $target_triple_flags $sysroot_flags $syscall_compat_flags {{defines}} {{include_dirs}} {{cflags}} {{cflags_c}} -c {{source}} -o {{output}}"
     depsformat = "gcc"
     description = "CC {{output}}"
     outputs =
@@ -38,7 +41,7 @@ toolchain("custom") {
 
   tool("cxx") {
     depfile = "{{output}}.d"
-    command = "$cxx -MD -MF $depfile $target_triple_flags $sysroot_flags {{defines}} {{include_dirs}} {{cflags}} {{cflags_cc}} -c {{source}} -o {{output}}"
+    command = "$cxx -MD -MF $depfile $target_triple_flags $sysroot_flags $syscall_compat_flags {{defines}} {{include_dirs}} {{cflags}} {{cflags_cc}} -c {{source}} -o {{output}}"
     depsformat = "gcc"
     description = "CXX {{output}}"
     outputs =
@@ -47,7 +50,7 @@ toolchain("custom") {
 
   tool("asm") {
     depfile = "{{output}}.d"
-    command = "$cc -MD -MF $depfile $target_triple_flags $sysroot_flags {{defines}} {{include_dirs}} {{asmflags}} {{cflags}} {{cflags_c}} -c {{source}} -o {{output}}"
+    command = "$cc -MD -MF $depfile $target_triple_flags $sysroot_flags $syscall_compat_flags {{defines}} {{include_dirs}} {{asmflags}} {{cflags}} {{cflags_c}} -c {{source}} -o {{output}}"
     depsformat = "gcc"
     description = "ASM {{output}}"
     outputs =

engine/src/build/toolchain/custom/tizen_syscall_compat.h

@@ -0,0 +1,32 @@
+// Copyright 2026 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BUILD_TOOLCHAIN_CUSTOM_TIZEN_SYSCALL_COMPAT_H_
+#define BUILD_TOOLCHAIN_CUSTOM_TIZEN_SYSCALL_COMPAT_H_
+
+// Some Tizen cross-compilation sysroots do not expose __NR_getrandom even
+// though the target Linux ABI supports the syscall. Dart's crypto_linux.cc
+// references the syscall number directly, so provide the missing arch-specific
+// values for custom Linux toolchains without modifying third_party sources.
+#if defined(__linux__)
+
+#if defined(__i386__) && !defined(__NR_getrandom)
+#define __NR_getrandom 355
+#endif
+
+#if defined(__x86_64__) && !defined(__NR_getrandom)
+#define __NR_getrandom 318
+#endif
+
+#if defined(__arm__) && !defined(__NR_getrandom)
+#define __NR_getrandom 384
+#endif
+
+#if defined(__aarch64__) && !defined(__NR_getrandom)
+#define __NR_getrandom 278
+#endif
+
+#endif  // defined(__linux__)
+
+#endif  // BUILD_TOOLCHAIN_CUSTOM_TIZEN_SYSCALL_COMPAT_H_