When building for 64-bit ARM, some symbol references may be resolved
via an "adrp" instruction (to obtain the start of the 4kB page
containing the symbol) and a separate 12-bit offset.  For example
(taken from the GNU assembler documentation):

  adrp x0, foo
  ldr  x0, [x0, #:lo12:foo]

We occasionally refer to symbols defined via mechanisms that are not
directly visible to gcc.  For example:

  extern char some_magic_symbol[];
  __asm__ ( ".equ some_magic_symbol, some_magic_expression" );

The subsequent use of the ":lo12:" prefix on such magically-defined
symbols triggers an assertion failure in the assembler.

This problem seems to affect only "private_key_len" in the current
codebase.  Fix by storing this value as static data; this avoids the
need to provide the value as a literal within the instruction stream,
and so avoids the problematic use of the ":lo12:" prefix.

Signed-off-by: Michael Brown <mcb30@ipxe.org>