meson: map powerpc64 TARGET_ARCH to ppc64 for the cross file

Meson uses 'ppc64' for 64 bit powerpc. Issue came up while
building systemd for MACHINE that uses ppc64e5500 tune.

(From OE-Core rev: eccd5414c37be26df63a90154c1808f6f5618b7d)

Signed-off-by: Victor Kamensky <kamensky@cisco.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/meson.bbclass

@@ -52,6 +52,8 @@ def meson_cpu_family(var, d):
     arch = d.getVar(var)
     if arch == 'powerpc':
         return 'ppc'
+    elif arch == 'powerpc64':
+        return 'ppc64'
     elif arch == 'mipsel':
         return 'mips'
     elif re.match(r"i[3-6]86", arch):