[efi] Fix off-by-one error in ARM .bss zeroing code in loader's _start

__bss_end should not be included in .bss zeroing code. Otherwise first 4
bytes of the section that follows .bss (in loader's case it's .sdata) are
overwritten by zero.

Reviewed by: andrew
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D9108