aarch64: replace asm source file for __set_thread_area with inline asm

this change both aligns with the intended future direction for most
assembly usage, and makes it possible to add arch-specific setup logic
based on hwcaps like we have for 32-bit arm.

src/thread/aarch64/__set_thread_area.c

@@ -0,0 +1,5 @@
+int __set_thread_area(void *p)
+{
+	__asm__ __volatile__ ("msr tpidr_el0,%0" : : "r"(p) : "memory");
+	return 0;
+}

src/thread/aarch64/__set_thread_area.s

@@ -1,7 +0,0 @@
-.global __set_thread_area
-.hidden __set_thread_area
-.type   __set_thread_area,@function
-__set_thread_area:
-	msr tpidr_el0,x0
-	mov w0,#0
-	ret