[embedded] Pass all default stdlib SWIFT_COMPILE_FLAGS to the embedded stdlib build too

[kubamracek]

This resolves a problem that currently the macOS toolchain CI job is excluding the embedded Swift.swiftmodule from the nightly toolchain because recursive lipo doesn't expect the x86-compiler-produced .swiftmodule and arm64-compiler-produced .swiftmodule to differ, but they do (because they embed file paths because of a missing -enable-experimental-concise-pound-file flag):

-- Warning: non-executable source files are different, skipping:
./macosx-arm64/Applications/Xcode.app/Contents/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2023-10-08-a.xctoolchain/usr/lib/swift/embedded/Swift.swiftmodule/armv7-apple-none-macho.swiftmodule
./macosx-x86_64/Applications/Xcode.app/Contents/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2023-10-08-a.xctoolchain/usr/lib/swift/embedded/Swift.swiftmodule/armv7-apple-none-macho.swiftmodule
-- Warning: non-executable source files are different, skipping:
./macosx-arm64/Applications/Xcode.app/Contents/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2023-10-08-a.xctoolchain/usr/lib/swift/embedded/Swift.swiftmodule/arm64-apple-macos.swiftmodule
./macosx-x86_64/Applications/Xcode.app/Contents/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2023-10-08-a.xctoolchain/usr/lib/swift/embedded/Swift.swiftmodule/arm64-apple-macos.swiftmodule
-- Warning: non-executable source files are different, skipping:
./macosx-arm64/Applications/Xcode.app/Contents/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2023-10-08-a.xctoolchain/usr/lib/swift/embedded/Swift.swiftmodule/arm64-apple-none-macho.swiftmodule
./macosx-x86_64/Applications/Xcode.app/Contents/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2023-10-08-a.xctoolchain/usr/lib/swift/embedded/Swift.swiftmodule/arm64-apple-none-macho.swiftmodule
-- Warning: non-executable source files are different, skipping:
./macosx-arm64/Applications/Xcode.app/Contents/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2023-10-08-a.xctoolchain/usr/lib/swift/embedded/Swift.swiftmodule/arm64e-apple-macos.swiftmodule
./macosx-x86_64/Applications/Xcode.app/Contents/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2023-10-08-a.xctoolchain/usr/lib/swift/embedded/Swift.swiftmodule/arm64e-apple-macos.swiftmodule
-- Warning: non-executable source files are different, skipping:
./macosx-arm64/Applications/Xcode.app/Contents/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2023-10-08-a.xctoolchain/usr/lib/swift/embedded/Swift.swiftmodule/x86_64-apple-macos.swiftmodule
./macosx-x86_64/Applications/Xcode.app/Contents/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2023-10-08-a.xctoolchain/usr/lib/swift/embedded/Swift.swiftmodule/x86_64-apple-macos.swiftmodule

[kubamracek]

@swift-ci please test