Revert "[arch][arm64][mmio] add 'Z' to the mmio write accessor inline…

… asm"

Sadly this doesn't really work in all situations and only happens to
work with gcc + binutils for 32bit accesses, presumably because gnu as
replaces a literal 0 with wzr.

Clang doesn't understand it at all.

This reverts commit 6c14941.

arch/arm64/include/arch/reg.h

@@ -39,11 +39,11 @@
 })
 
 #define _ARCH_MMIO_WRITE8(addr, val) \
-    __asm__ volatile("strb %w1, %0" : "=m"(*(addr)) : "Zr"(val) : "memory")
+    __asm__ volatile("strb %w1, %0" : "=m"(*(addr)) : "r"(val) : "memory")
 #define _ARCH_MMIO_WRITE16(addr, val) \
-    __asm__ volatile("strh %w1, %0" : "=m"(*(addr)): "Zr"(val) : "memory")
+    __asm__ volatile("strh %w1, %0" : "=m"(*(addr)): "r"(val) : "memory")
 #define _ARCH_MMIO_WRITE32(addr, val) \
-    __asm__ volatile("str %w1, %0" : "=m"(*(addr)) : "Zr"(val) : "memory")
+    __asm__ volatile("str %w1, %0" : "=m"(*(addr)) : "r"(val) : "memory")
 #define _ARCH_MMIO_WRITE64(addr, val) \
-    __asm__ volatile("str %1, %0" : "=m"(*(addr)) : "Zr"(val) : "memory")
+    __asm__ volatile("str %1, %0" : "=m"(*(addr)) : "r"(val) : "memory")