[Bug 224008] UBLDR doesn't save registers correctly in start.S

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=224008

            Bug ID: 224008
           Summary: UBLDR doesn't save registers correctly in start.S
           Product: Base System
           Version: 11.1-RELEASE
          Hardware: arm
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: arm
          Assignee: freebsd-arm at FreeBSD.org
          Reporter: parakleta at darkreality.org

Created attachment 188440
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=188440&action=edit
Save and Restore U-Boot registers.

Using `ubldr.bin` I was getting intermittent behaviour depending on which
options I compiled into U-Boot.  Eventually I discovered that the issue is that
the entry point in `sys/boot/arm/uboot/start.S` is not correctly saving and
restoring registers.  Most importantly the r9 register is the base of the
global data and this is not typically a callee-saved register, but must be for
U-Boot.

There is a change to this file on April 2 by "ian" which saves the argument and
return address but it doesn't go far enough.  My patch is to applied to the
state before that patch.

Essentially the important caller-saved registers are pushed (r0, r1, r9, lr)
before the relocation call, and popped after.  Then r8/r9 are saved as usual
for the syscall trampoline, and lr is stored in r8 (now free) as a callee-saved
value before calling into `main`.

The call to `main` can no longer be a tail call because we must restore r9
especially after main returns (although since we have used r8 to hold lr we
must also restore this).

I don't know how this affects the ELF version, but this patch is critical for
the correct functioning of the binary version of UBLDR.

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