arm: build: Support -mcpu= on ARM and ARM64

src/build/config/arm.gni

@@ -37,6 +37,8 @@ if (current_cpu == "arm" || v8_current_cpu == "arm") {
     # Thumb is a reduced instruction set available on some ARM processors that
     # has increased code density.
     arm_use_thumb = true
+
+    arm_cpu = ""
   }
 
   if (current_os == "android" || target_os == "android") {
@@ -122,6 +124,9 @@ if (current_cpu == "arm" || v8_current_cpu == "arm") {
   # arm64 supports only "hard".
   arm_float_abi = "hard"
   arm_use_neon = true
+  declare_args() {
+    arm_cpu = ""
+  }
   declare_args() {
     # Enables the new Armv8 branch protection features. Valid strings are:
     # - "pac": Enables Pointer Authentication Code (PAC, featured in Armv8.3)

src/build/config/compiler/BUILD.gn

@@ -871,7 +871,11 @@ config("compiler") {
     # TODO(pcc): The contents of .ARM.attributes should be based on the
     # -march flag passed at compile time (see llvm.org/pr36291).
     if (current_cpu == "arm") {
-      ldflags += [ "-march=$arm_arch" ]
+      if (arm_cpu != "") {
+        ldflags += [ "-mcpu=$arm_cpu" ]
+      } else {
+        ldflags += [ "-march=$arm_arch" ]
+      }
     }
   }
 
@@ -1229,9 +1233,14 @@ config("compiler_cpu_abi") {
       }
       if (!is_nacl) {
         cflags += [
-          "-march=$arm_arch",
           "-mfloat-abi=$arm_float_abi",
         ]
+        if (arm_cpu != "") {
+          cflags += [ "-mcpu=$arm_cpu" ]
+        } else {
+          cflags += [ "-march=$arm_arch" ]
+        }
+        ldflags += [ "-mfloat-abi=$arm_float_abi" ]
       }
       if (arm_tune != "") {
         cflags += [ "-mtune=$arm_tune" ]
@@ -1242,6 +1251,9 @@ config("compiler_cpu_abi") {
         cflags += [ "--target=aarch64-linux-gnu" ]
         ldflags += [ "--target=aarch64-linux-gnu" ]
       }
+      if (arm_cpu != "") {
+        cflags += [ "-mcpu=$arm_cpu" ]
+      }
     } else if (current_cpu == "mipsel" && !is_nacl) {
       ldflags += [ "-Wl,--hash-style=sysv" ]
       if (custom_toolchain == "") {