[build-script] Link Objective-C runtime for XCTest

The XCTest CI is currently broken for OS X. Invoking `xcodebuild`, even
when setting the correct `SWIFT_EXEC`, was unable to build Foundation.
The problem was that a flag to link the Objective-C runtime was not being set
as it would be when building with a Swift toolchain.

Set the flag to get the builds passing. The XCTest CI presets should
pass on all platforms with this change.

Author: modocache

utils/build-script-impl

@@ -1916,6 +1916,7 @@ for deployment_target in "${HOST_TARGET}" "${CROSS_TOOLS_DEPLOYMENT_TARGETS[@]}"
                         -workspace "${XCTEST_SOURCE_DIR}"/XCTest.xcworkspace \
                         -scheme SwiftXCTest \
                         SWIFT_EXEC="${SWIFTC_BIN}" \
+                        SWIFT_LINK_OBJC_RUNTIME=YES \
                         SKIP_INSTALL=NO \
                         DEPLOYMENT_LOCATION=YES \
                         DSTROOT="${XCTEST_BUILD_DIR}" \
@@ -2183,7 +2184,8 @@ for deployment_target in "${STDLIB_DEPLOYMENT_TARGETS[@]}"; do
                     xcodebuild \
                         -workspace "${XCTEST_SOURCE_DIR}"/XCTest.xcworkspace \
                         -scheme SwiftXCTestFunctionalTests \
-                        SWIFT_EXEC="${SWIFTC_BIN}"
+                        SWIFT_EXEC="${SWIFTC_BIN}" \
+                        SWIFT_LINK_OBJC_RUNTIME=YES
                     { set +x; } 2>/dev/null
                 else
                     FOUNDATION_BUILD_DIR=$(build_directory ${deployment_target} foundation)