[build] Add support for is_ios in BUILD.gn

Builds for an iOS target were left unhandled in the v8_libbase
component inside BUILD.gn. This adds the appropriate source files to
the build for iOS targets.

Tbr: machenbach@chromium.org
Bug: v8:9140
Change-Id: I853e9a39727d80f0f281c8ac2da4eeb465e188e6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1569430Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60871}

BUILD.gn

@@ -3518,7 +3518,7 @@ v8_component("v8_libbase") {
       "src/base/debug/stack_trace_fuchsia.cc",
       "src/base/platform/platform-fuchsia.cc",
     ]
-  } else if (is_mac) {
+  } else if (is_mac || is_ios) {
     sources += [
       "src/base/debug/stack_trace_posix.cc",
       "src/base/platform/platform-macos.cc",