x/net/internal/quic: TestUDPECN consistently failing with EINVAL on Android

[bcmills]

Go version

0784fd1

Output of go env in your module/workspace:

N/A

What did you do?

https://build.golang.org/log/6433940bdb66eb651712997e88c329a447a33e15

What did you see happen?

--- FAIL: TestUDPECN (0.01s)
    --- FAIL: TestUDPECN/udp4/udp/unspec (0.00s)
        udp_test.go:89: Write: write udp4 127.0.0.1:42859->127.0.0.1:44007: sendmsg: invalid argument
    --- FAIL: TestUDPECN/udp4/udp4/unspec (0.00s)
        udp_test.go:89: Write: write udp4 127.0.0.1:55776->127.0.0.1:41537: sendmsg: invalid argument
    --- FAIL: TestUDPECN/udp4/udp4/spec (0.00s)
        udp_test.go:89: Write: write udp4 127.0.0.1:45943->127.0.0.1:55286: sendmsg: invalid argument
FAIL
FAIL	golang.org/x/net/quic	22.172s

What did you expect to see?

Tests passing.

The symptoms are superficially similar to #65907, but in this case the affected builders are little-endian Android.

(attn @dneil, @jba, @golang/android)