build fails on macOS 10.15 Beta

[Jiawen Geng]

After I upgrade macOS, I can't build V8 any more

```

gn gen --export-compile-commands out/x86.release

tools/dev/gm.py x64.release

```

```console

[11/1237] CXX obj/torque_base/utils.o

FAILED: obj/torque_base/utils.o 

../../third_party/llvm-build/Release+Asserts/bin/clang++ -MMD -MF obj/torque_base/utils.o.d -DNO_TCMALLOC -DFULL_SAFE_BROWSING -DSAFE_BROWSING_CSD -DSAFE_BROWSING_DB_LOCAL -DCHROMIUM_BUILD -D_LIBCPP_HAS_NO_ALIGNED_ALLOCATION -DCR_XCODE_VERSION=1021 -DCR_CLANG_REVISION=\"361565-342571e8-1\" -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D_FORTIFY_SOURCE=2 -D_LIBCPP_ABI_UNSTABLE -D_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS -D_LIBCXXABI_DISABLE_VISIBILITY_ANNOTATIONS -D_LIBCPP_ENABLE_NODISCARD -DCR_LIBCXX_REVISION=361348 -D__ASSERT_MACROS_DEFINE_VERSIONS_WITHOUT_UNDERSCORE=0 -DNDEBUG -DNVALGRIND -DDYNAMIC_ANNOTATIONS_ENABLED=0 -DENABLE_DISASSEMBLER -DV8_TYPED_ARRAY_MAX_SIZE_IN_HEAP=64 -DENABLE_GDB_JIT_INTERFACE -DENABLE_MINOR_MC -DOBJECT_PRINT -DVERIFY_HEAP -DV8_INTL_SUPPORT -DV8_USE_SNAPSHOT -DV8_USE_EXTERNAL_STARTUP_DATA -DV8_CONCURRENT_MARKING -DV8_EMBEDDED_BUILTINS -DV8_WIN64_UNWINDING_INFO -DV8_DEPRECATION_WARNINGS -DV8_IMMINENT_DEPRECATION_WARNINGS -DV8_TARGET_ARCH_X64 -DDISABLE_UNTRUSTED_CODE_MITIGATIONS -DV8_DEPRECATION_WARNINGS -DV8_IMMINENT_DEPRECATION_WARNINGS -I../.. -Igen -I../.. -Igen -fno-strict-aliasing -fstack-protector -fcolor-diagnostics -fmerge-all-constants -fcrash-diagnostics-dir=../../tools/clang/crashreports -Xclang -mllvm -Xclang -instcombine-lower-dbg-declare=0 -fcomplete-member-pointers -arch x86_64 -Wno-builtin-macro-redefined -D__DATE__= -D__TIME__= -D__TIMESTAMP__= -no-canonical-prefixes -Wall -Werror -Wextra -Wimplicit-fallthrough -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 -fno-omit-frame-pointer -gdwarf-4 -g2 -isysroot ../../../../../../../Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk -mmacosx-version-min=10.10.0 -Wheader-hygiene -Wstring-conversion -Wtautological-overlap-compare -Wmissing-field-initializers -Wunreachable-code -Wshorten-64-to-32 -O3 -fvisibility=default -Wexit-time-destructors -std=c++14 -stdlib=libc++ -nostdinc++ -isystem../../buildtools/third_party/libc++/trunk/include -isystem../../buildtools/third_party/libc++abi/trunk/include -fexceptions -frtti -c ../../src/torque/utils.cc -o obj/torque_base/utils.o

clang++: error: no such sysroot directory: '../../../../../../../Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk' [-Werror,-Wmissing-sysroot]

ninja: build stopped: subcommand failed.

Error! - V8 compilation finished with errors.

```