Fix cross-compile build failure.
In Build-FoundationMacros, use the build Swift SDK for the build case
and when cross-compiling, use the swift syntax build from the build
arch binary cache as opposed to the host arch binary cache or else the
architecture mismatch will happen.
Cherrypick commit https://github.com/swiftlang/swift/pull/75970/commits/3054eab6b084ed1223780c512fb2fac69c9bdee9
