[RFC PATCH 14/18] tests: add aarch64 arm to start.S

The userland test stubs run as freestanding multiboot modules: the
kernel exec path hands them control at _start with argc/argv/envp
laid out sysv-style on the stack, and they call c_start(sp) to find
their arguments.  The i386 and x86_64 arms already implement this;
add the aarch64 equivalent (mov x0, sp; bl c_start) so test modules
can be cross-compiled with aarch64-unknown-none-elf-gcc and entered
by Bugaev's pcb/exec path.

bl rather than a tail-call b keeps the saved-LR contract intact in
case anything unwinds out of the userland prelude.
---
 tests/start.S | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/tests/start.S b/tests/start.S
index 15970fb9..71e055ab 100644
--- a/tests/start.S
+++ b/tests/start.S
@@ -26,5 +26,15 @@ _start:
         movq    %rsp,%rdi
         callq   c_start
 #endif /* __x86_64__ */
+#ifdef __aarch64__
+        /*
+         * Kernel hands us argc/argv/envp on the stack (sysv-style);
+         * pass sp through x0 as c_start's first argument (per AAPCS).
+         * bl rather than a tail-call keeps the saved-LR contract
+         * intact in case anything unwinds.
+         */
+        mov     x0, sp
+        bl      c_start
+#endif /* __aarch64__ */
 
 	.section .note.GNU-stack,"",%progbits
-- 
2.54.0