Don't add -arch explictly to stdlib compile flags on Darwin

[edymtt]

Rely instead on setting the OSX_ARCHITECTURES property (#38415)

Address rdar://96087734

[edymtt]

@swift-ci please test

[edymtt]

@swift-ci please build toolchain

[compnerd]

Probably a good idea to check that there aren't other cases that we forgot about.

[etcwilde]

The windows breakage appears to be a missing dependency edge in a circular dependency between swiftAST and swiftBasic. libswiftAST.a contains the InFlightDiagnostic::flush definition, but isn't linked back into libswiftBasic.a, which uses it. Well that's gross.

[etcwilde]

We will probably need #61001 to make the windows build work.
Someone should probably pull the diagnostic bits into swiftBasic and break this link cycle, but later.

[edymtt]

@swift-ci please test

[edymtt]

@swift-ci please build toolchain

[edymtt]

From a quick search in CMakeLists.txt/*.cmake files I don't see any other place in which we manually append -arch parameters.