[Runtime] Fix generation of .lnk files for static linking.

[al45tair]

Use the new SWIFT_SDK_<sdk>_STATIC_LINKING_SUPPORTED and _STATIC_ONLY flags instead of hardcoding support for Linux and WASI.

Also, use the _LIB_SUBDIR variable rather than lowercasing the SDK.

rdar://123504757

[al45tair]

Explanation: SwiftPM needs .lnk files when statically linking to determine which libraries it should link by default. These need updating to support the fully static SDK.
Original PR: #71843
Risk: Low. Only affects static linking, and should have the same behaviour as existing build script code for Linux and WASI.
Reviewed by: @MaxDesiatov
Resolves: rdar://123504757
Tests: There are some static linking tests in the repo already; the changes that affect the new static SDK are currently tested manually (by building some code with SwiftPM), but there will be CI at some point soon.

[al45tair]

@swift-ci Please test