allocator-test: fix trusted stack revocation probe

Ensure that the compiler loads the to be revoked pointer into a register
prior to releasing the main thread.  Otherwise, there is a chance that
we never hold an unrevoked copy in the register file.

Embarrassing fix to
https://github.com/CHERIoT-Platform/cheriot-rtos/pull/422

tests/allocator-test.cc

diff --git a/tests/allocator-test.cc b/tests/allocator-test.cc
index ae71d5d..87af6c3 100644
--- a/tests/allocator-test.cc
+++ b/tests/allocator-test.cc
@@ -126,14 +126,12 @@
 			async([=]() {
 				int ptag, scratch;
 
-				// Tell the main thread to go
-				state = 0;
-
 				/*
-				 * Busy spin, ensuring that our test pointer is in a register
-				 * throughout, then get its tag.
+				 * Release the main thread, then busy spin, ensuring that our
+				 * test pointer is in a register throughout, then get its tag.
 				 */
-				__asm__ volatile("1:\n"
+				__asm__ volatile("csw zero, 0(%[state])\n"
+				                 "1:\n"
 				                 "clw %[scratch], 0(%[state])\n"
 				                 "beqz %[scratch], 1b\n"
 				                 "cgettag %[out], %[p]\n"