Issue #21085: add configure check for siginfo_t.si_band

Patch by Masayuki Yamamoto, reviewed and rebased by Erik Bray.

This is a first step on the long road toward resupporting Cygwin, which does
not provide siginfo_t.si_band.
diff --git a/Misc/ACKS b/Misc/ACKS
index a5465a2..467f3aa 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -1671,6 +1671,7 @@
 Arnon Yaari
 Alakshendra Yadav
 Hirokazu Yamamoto
+Masayuki Yamamoto
 Ka-Ping Yee
 Chi Hsuan Yen
 Jason Yeo