[mac] Remove definitions for unsupported toolchains

This change removes the definitions for the following toolchains:

  - arm toolchains because macOS never had support for 32-bit arm cpus

    //build/toolchain/mac:clang_arm
    //build/toolchain/mac:clang_arm_for_rust_host_build_tools
    //build/toolchain/mac:clang_arm_host_with_system_allocator
    //build/toolchain/mac:clang_arm_with_system_allocator

  - x86 toolchains because macOS dropped support for 32-bit intel cpus
    in 2019 with macOS Catalina

    //build/toolchain/mac:clang_x86
    //build/toolchain/mac:clang_x86_for_rust_host_build_tools
    //build/toolchain/mac:clang_x86_host_with_system_allocator
    //build/toolchain/mac:clang_x86_with_system_allocator
    //build/toolchain/mac:clang_x86_v8_arm
    //build/toolchain/mac:clang_x86_v8_arm_for_rust_host_build_tools
    //build/toolchain/mac:clang_x86_v8_arm_host_with_system_allocator
    //build/toolchain/mac:clang_x86_v8_arm_with_system_allocator
    //build/toolchain/mac:clang_x86_v8_mipsel
    //build/toolchain/mac:clang_x86_v8_mipsel_for_rust_host_build_tools
    //build/toolchain/mac:clang_x86_v8_mipsel_host_with_system_allocator
    //build/toolchain/mac:clang_x86_v8_mipsel_with_system_allocator

Note that trying to build with any of those toolchain would result in
assertion failures in many places in other BUILD.gn files (e.g. the
rust toolchain only supports x64 and arm64 cpus, ...).

Bug: 452018499
Change-Id: I6d1313e4b5a1a64b54ac3afe9a3a710ec090fb82
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7041722
Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1530065}
NOKEYCHECK=True
GitOrigin-RevId: 7e9b2780f03a3a5751ccc947cb3d0cb77067e76b

apple/toolchain.gni

diff --git a/apple/toolchain.gni b/apple/toolchain.gni
index 6409d34..c5e065c 100644
--- a/apple/toolchain.gni
+++ b/apple/toolchain.gni
@@ -251,18 +251,8 @@
         _archs_mapping_os_cpu = _archs_mapping_os[toolchain_args.current_cpu]
         _dsym_arch = _archs_mapping_os_cpu.dsym_arch
       } else {
-        # There are toolchain defined with combination of os and cpu that are
-        # no longer supported. Avoid failing `gn gen` with error in that case
-        # by defining _dsym_arch to some value that will cause a failure if
-        # used at build time.
-        #
-        # TODO(crbug.com/452018499): Remove once the toolchains no longer exist.
         not_needed([ "_archs_mapping_os" ])
-        _dsym_arch =
-            "invalid__this_name_is_longer_than_NAME_MAX_so_that_it_cant_be_us" +
-            "ed__0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwx" +
-            "yz_0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxy" +
-            "z_0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
+        assert("cpu unsupported ${toolchain_args.current_cpu}")
       }
 
       dsym_output_dir =

mac/BUILD.gn

diff --git a/mac/BUILD.gn b/mac/BUILD.gn
index d8543c4..0063a0f 100644
--- a/mac/BUILD.gn
+++ b/mac/BUILD.gn
@@ -52,12 +52,6 @@
   }
 }
 
-mac_toolchain("clang_arm") {
-  toolchain_args = {
-    current_cpu = "arm"
-  }
-}
-
 mac_toolchain("clang_arm64") {
   toolchain_args = {
     current_cpu = "arm64"
@@ -70,26 +64,6 @@
   }
 }
 
-mac_toolchain("clang_x86") {
-  toolchain_args = {
-    current_cpu = "x86"
-  }
-}
-
-mac_toolchain("clang_x86_v8_arm") {
-  toolchain_args = {
-    current_cpu = "x86"
-    v8_current_cpu = "arm"
-  }
-}
-
-mac_toolchain("clang_x86_v8_mipsel") {
-  toolchain_args = {
-    current_cpu = "x86"
-    v8_current_cpu = "mipsel"
-  }
-}
-
 mac_toolchain("clang_x64_v8_arm64") {
   toolchain_args = {
     current_cpu = "x64"