affinity_posix should get this patch (for Android/Termux)

[HJarausch]

I have adapted a recent patch to threadlocalstorage.nim from Nim-Devel to affinity_posix.nim.
Please apply the following patch to affinity_posix.nim.

--- affinity_posix.nim.ORIG	2021-04-07 15:15:30.550835730 +0200
+++ affinity_posix.nim	2021-04-07 16:51:07.136049648 +0200
@@ -35,17 +35,31 @@
 # Nim doesn't allow the main thread to set its own affinity
 
 proc set_thread_affinity(t: Pthread, cpu: int32) {.inline.}=
-  when defined(osx) or defined(android):
+  when defined(osx):
     {.warning: "To improve performance we should pin threads to cores.\n" &
-                "This is not possible with MacOS or Android.".}
-    # Note: on Android it's even more complex due to the Big.Little architecture
-    #       with cores with different performance profiles to save on battery
-  else:
+                "This is not possible with MacOS.".}
+  else :
     var cpuset {.noinit.}: CpuSet
 
     cpu_zero(cpuset)
     cpu_set(cpu, cpuset)
-    pthread_setaffinity_np(t, sizeof(CpuSet), cpuset)
+  
+    if  defined(android) :
+      # libc of android doesn't implement pthread_setaffinity_np,
+      # it exposes pthread_gettid_np though, so we can use that in combination
+      # with sched_setaffinity to set the thread affinity.
+      type Pid {.importc: "pid_t", header: "<sys/types.h>".} = int32 # From posix_other.nim
+
+      proc setAffinityTID(tid: Pid; setsize: csize_t; s: var CpuSet) {.
+        importc: "sched_setaffinity", header: "<sched.h>".}
+
+      proc pthread_gettid_np(thread: Pthread): Pid {.
+        importc: "pthread_gettid_np", header: "<pthread.h>".}
+
+      setAffinityTID(pthread_gettid_np(t), csize_t(sizeof(CpuSet)), cpuset)
+
+    else:
+      pthread_setaffinity_np(t, sizeof(CpuSet), cpuset)
 
 proc pinToCpu*(cpu: int32) {.inline.} =
   ## Set the affinity of the main thread (the calling thread)