The ISB instruction isn't available in ARMv5 or v6, so
guard it's use to fix the build on qemuarmv5.

Upstream-Status: Pending
Signed-off-by: Ross Burton <ross.burton@arm.com>

diff --git a/js/src/jit/arm/Architecture-arm.cpp b/js/src/jit/arm/Architecture-arm.cpp
--- a/js/src/jit/arm/Architecture-arm.cpp	2024-01-18 17:31:32.078718197 +0000
+++ b/js/src/jit/arm/Architecture-arm.cpp	2024-01-18 18:00:16.738921445 +0000
@@ -529,7 +529,9 @@
 void FlushExecutionContext() {
 #ifndef JS_SIMULATOR_ARM
+#if __ARM_ARCH >= 7
   // Ensure that any instructions already in the pipeline are discarded and
   // reloaded from the icache.
   asm volatile("isb\n" : : : "memory");
+#endif
 #else
   // We assume the icache flushing routines on other platforms take care of this