pc-bios/s390-ccw: Don't use __bss_start with the "larl" instruction

start.S currently cannot be compiled with Clang 16 and binutils 2.40:

 ld: start.o(.text+0x8): misaligned symbol `__bss_start' (0xc1e5) for
     relocation R_390_PC32DBL

According to the built-in linker script of ld, the symbol __bss_start
can actually point *before* the .bss section and does not need to have
any alignment, so in certain situations (like when using the internal
assembler of Clang), the __bss_start symbol can indeed be unaligned
and thus it is not suitable for being used with the "larl" instruction
that needs an address that is at least aligned to halfwords.
The problem went unnoticed so far since binutils <= 2.39 did not
check the alignment, but starting with binutils 2.40, such unaligned
addresses are now refused.

Fix it by loading the address indirectly instead.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2216662
Reported-by: Miroslav Rezanina <mrezanin@redhat.com>
Suggested-by:  Andreas Krebbel <andreas.krebbel@de.ibm.com>
Message-Id: <20230629104821.194859-8-thuth@redhat.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

pc-bios/s390-ccw/start.S

@@ -19,7 +19,8 @@ _start:
     larl    %r15,stack + STACK_SIZE - STACK_FRAME_SIZE   /* Set up stack */
 
     /* clear bss */
-    larl    %r2,__bss_start
+    larl    %r2,bss_start_literal   /* __bss_start might be unaligned ... */
+    lg      %r2,0(%r2)              /* ... so load it indirectly */
     larl    %r3,_end
     slgr    %r3,%r2    /* get sizeof bss */
     ltgr    %r3,%r3    /* bss empty? */
@@ -45,7 +46,6 @@ done:
 memsetxc:
     xc      0(1,%r1),0(%r1)
 
-
 /*
  * void disabled_wait(void)
  *
@@ -113,6 +113,8 @@ io_new_code:
     br      %r14
 
     .align  8
+bss_start_literal:
+    .quad   __bss_start
 disabled_wait_psw:
     .quad   0x0002000180000000,0x0000000000000000
 enabled_wait_psw: