musl ldso issues a membarrier when setting up TLS

--- firefox-68.7.0/security/sandbox/linux/SandboxFilter.cpp	2020-04-03 19:30:03.000000000 +0000
+++ firefox-68.7.0/security/sandbox/linux/SandboxFilter.cpp	2020-04-19 04:59:30.280000000 +0000
@@ -529,6 +529,7 @@ class SandboxPolicyCommon : public Sandb
 
         // ipc::Shmem; also, glibc when creating threads:
       case __NR_mprotect:
+      case __NR_membarrier:
         return Allow();
 
         // madvise hints used by malloc; see bug 1303813 and bug 1364533