fix build (by avoiding purego) on 32-bit GOARCH builds

This makes those architectures effectively always build in "nodynamic"
mode without having to set the build tag.

Signed-off-by: Brad Fitzpatrick <brad@danga.com>

Author: bradfitz

decode_dynamic.go

@@ -1,4 +1,4 @@
-//go:build (unix || darwin || windows) && !nodynamic
+//go:build (unix || darwin || windows) && !(nodynamic || arm || 386 || mips || mipsle)
 
 package heic
 

purego_other.go

@@ -1,4 +1,4 @@
-//go:build (!unix && !darwin && !windows) || nodynamic
+//go:build (!unix && !darwin && !windows) || nodynamic || arm || 386 || mips || mipsle
 
 package heic
 

purego_unix.go

@@ -1,4 +1,4 @@
-//go:build unix && !darwin && !nodynamic
+//go:build unix && !darwin && !(nodynamic || arm || 386 || mips || mipsle)
 
 package heic