fix(build): disable Matrix gateway import on freebsd/arm

Exclude the Matrix gateway shim from freebsd/arm builds because
modernc.org/libc currently fails to compile on that target.
Document the upstream 32-bit FreeBSD codegen mismatch as well.

pkg/gateway/channel_matrix.go

@@ -1,4 +1,4 @@
-//go:build !mipsle && !netbsd
+//go:build !mipsle && !netbsd && !(freebsd && arm)
 
 package gateway
 
@@ -12,6 +12,9 @@ import (
 	// - netbsd/*: modernc.org/sqlite v1.46.1 fails to compile due to broken
 	//   generated mutex code on NetBSD (for example sqlite_netbsd_amd64.go calls
 	//   mu.enter/mu.leave, but the generated mutex type does not define them).
+	// - freebsd/arm: modernc.org/libc v1.67.6 fails to compile due to broken
+	//   generated 32-bit FreeBSD code (size_t/uint64 and int32/int64 mismatches
+	//   in libc_freebsd.go).
 	//
 	// This means Matrix is currently unavailable on those targets. The proper
 	// long-term fix is to split Matrix basic support from its E2EE/sqlite-backed