Fix T20638 on big-endian architectures

The test reads a 16 bit value from an array of 8 bit values. Naturally,
that leads to different values read on big-endian architectures than
on little-endian. In this case the value read is 0x8081 on big-endian
and 0x8180 on little endian. This patch changes the argument of the `and`
machop to mask bit 7 which is the only bit different. The test still checks
that bit 15 is zero, which was the original issue in #20638.

Fixes #20906.

testsuite/tests/cmm/should_run/machops/T20638.cmm

 test(bits64 buffer) {
-  return (%zx64(%shrl(bits16[buffer + (128 :: bits64)], (1 :: bits64))) & (64711 :: bits64));
+  return (%zx64(%shrl(bits16[buffer + (128 :: bits64)], (1 :: bits64))) & (65407 :: bits64));
 }
 

testsuite/tests/cmm/should_run/machops/T20638.stdout

-16576
+16448