[OpenMP] Add IBM Z as supported architecture in the runtime README

This architecture has been supported since
1111ef0,
but the README file had not been updated.

Author: uweigand

openmp/runtime/README.txt

@@ -52,6 +52,7 @@ Architectures Supported
 * Aarch64 (64-bit ARM) architecture
 * IBM(R) Power architecture (big endian)
 * IBM(R) Power architecture (little endian)
+* IBM(R) Z (s390x) architecture
 * MIPS and MIPS64 architecture
 * RISCV64 architecture
 * LoongArch64 architecture