m68k: Fix boot regression on machines with RAM at non-zero

My enhancement to store the initial mapping size for later reuse in commit
486df8bc4627bdfc032d11bedcd056cc5343ee62 ("m68k: Increase initial mapping
to 8 or 16 MiB if possible") broke booting on machines where RAM doesn't
start at address zero.

Use pc-relative addressing to fix this.

Reported-by: Andreas Schwab <schwab@linux-m68k.org>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Tested-by: Andreas Schwab <schwab@linux-m68k.org>