./build_all.sh stops at CXX obj/v8_libplatform/default-platform.o

まず build_all.sh と build_v8.sh を読み、その後、なぜビルドが third_party/llvm-build/Release+Asserts/bin/clang++ を必要としているのかを調査してください。必要な toolchain のパスを確認してビルドを再実行します。clang++ が見つからないことによる失敗を V8 のビルドが通過すれば完了です。

./build_v8.sh
~/ns-v8ios-runtime/v8 ~/ns-v8ios-runtime
Building for out.gn/x64-release
Done. Made 188 targets from 84 files in 4572ms
ninja: Entering directory `out.gn/x64-release'
[16/1764] CXX clang_x64/obj/torque_base/types.o
FAILED: clang_x64/obj/torque_base/types.o
../../third_party/llvm-build/Release+Asserts/bin/clang++ -MMD -MF clang_x64/obj/torque_base/types.o.d -DOFFICIAL_BUILD -D_LIBCPP_HAS_NO_ALIGNED_ALLOCATION -DCR_XCODE_VERSION=1150 -DCR_CLANG_REVISION="n356902-4e813bbd-1" -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D_FORTIFY_SOURCE=2 -D__ASSERT_MACROS_DEFINE_VERSIONS_WITHOUT_UNDERSCORES=0 -DNDEBUG -DNVALGRIND -DDYNAMIC_ANNOTATIONS_ENABLED=0 -DV8_TYPED_ARRAY_MAX_SIZE_IN_HEAP=64 -DENABLE_GDB_JIT_INTERFACE -DENABLE_MINOR_MC -DENABLE_HANDLE_ZAPPING -DV8_CONCURRENT_MARKING -DV8_ARRAY_BUFFER_EXTENSION -DV8_ENABLE_LAZY_SOURCE_POSITIONS -DV8_SHARED_RO_HEAP -DV8_WIN64_UNWINDING_INFO -DV8_ENABLE_REGEXP_INTERPRETER_THREADED_DISPATCH -DV8_SNAPSHOT_COMPRESSION -DV8_DEPRECATION_WARNINGS -DV8_IMMINENT_DEPRECATION_WARNINGS -DV8_TARGET_ARCH_X64 -DV8_HAVE_TARGET_OS -DV8_TARGET_OS_IOS -DDISABLE_UNTRUSTED_CODE_MITIGATIONS -DV8_DEPRECATION_WARNINGS -DV8_IMMINENT_DEPRECATION_WARNINGS -I../.. -Iclang_x64/gen -I../.. -I../../include -Iclang_x64/gen -fno-strict-aliasing -fstack-protector -fcolor-diagnostics -fmerge-all-constants -arch x86_64 -no-canonical-prefixes -Wall -Werror -Wextra -Wimplicit-fallthrough -Wunreachable-code -Wthread-safety -Wextra-semi -Wunguarded-availability -Wno-missing-field-initializers -Wno-unused-parameter -Wno-c++11-narrowing -Wno-unneeded-internal-declaration -Wno-undefined-var-template -Wno-ignored-pragma-optimize -Wno-implicit-int-float-conversion -Wno-final-dtor-non-final-class -Wno-builtin-assume-aligned-alignment -Wno-deprecated-copy -Wno-non-c-typedef-for-linkage -Wno-max-tokens -fno-omit-frame-pointer -g0 -isysroot ../../../../../../Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk -mmacosx-version-min=10.10.0 -fvisibility=hidden -Wheader-hygiene -Wstring-conversion -Wtautological-overlap-compare -Wno-error -Wmissing-field-initializers -Wunreachable-code -Wshorten-64-to-32 -O3 -Wexit-time-destructors -std=c++14 -fno-trigraphs -Wno-trigraphs -stdlib=libc++ -fvisibility-inlines-hidden -fexceptions -frtti -c ../../src/torque/types.cc -o clang_x64/obj/torque_base/types.o
/bin/sh: ../../third_party/llvm-build/Release+Asserts/bin/clang++: No such file or directory
[21/1764] CXX obj/v8_libplatform/default-platform.o
ninja: build stopped: subcommand failed.