Fix pmbr issues > 2TB

These issues have low impact because they require precise circumstances
to trigger one of them. The disk must be > 2 TiB in size and either:

• The primary GPT header is dammaged.
• The freebsd-boot partiton is located farther than the first 2 TiB of the disc and one of its sectors takes place at a lba value that makes the higher 32 bits of this very value change.

Errors and corrections folow:

• decl and incl don't affect CF, so replace with subl/addl $1
• repe uses %cx, so move size to it with movw
• moving a 64-bit value with %cx of 2 (should be 4) so addresses > 2TB will work.

PR: 233180
Reviewed by: imp@ (applied patch using description in bug)
Differential Revision: https://reviews.freebsd.org/D31100

(cherry picked from commit 0ca9f1d4a3b772036309fb1c14262ec77c674c5d)