No, it probably has nothing to do with my handling of --sysroot. The test executable produced by SwiftPM looks like it was linked properly:
$ [...]/llvm-objdump -p .build/i686-unknown-linux-android/debug/WorkspacePackageTests.xctest
.build/i686-unknown-linux-android/debug/WorkspacePackageTests.xctest: file format ELF32-i386
[...]
RUNPATH $ORIGIN
NEEDED libswiftSwiftOnoneSupport.so
NEEDED libswiftCore.so
NEEDED libFoundation.so
NEEDED libswiftGlibc.so
NEEDED libdl.so
NEEDED libm.so
NEEDED libswiftDispatch.so
NEEDED libdispatch.so
NEEDED libBlocksRuntime.so
NEEDED libFoundationXML.so
NEEDED libFoundationNetworking.so
NEEDED libXCTest.so
NEEDED libc.so
Whereas the libswiftSwiftOnoneSupport.so from the SDK contains a reference to lib/swift/android/i686/libswiftCore.so that looks out of place:
$ [...]/llvm-objdump -p .build/i686-unknown-linux-android/debug/libswiftSwiftOnoneSupport.so
.build/i686-unknown-linux-android/debug/libswiftSwiftOnoneSupport.so: file format ELF32-i386
[...]
NEEDED libm.so
NEEDED libdl.so
NEEDED liblog.so
NEEDED libc++_shared.so
NEEDED libicuin64.so
NEEDED libicuuc64.so
NEEDED lib/swift/android/i686/libswiftCore.so
NEEDED libc.so
At least libswiftGlibc.so also exhibits the same NEEDED entry: lib/swift/android/i686/libswiftCore.so