MFC r311888, r311890-r311891

r311888:
[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
Differential Revision: https://reviews.freebsd.org/D9108

r311890:
[efi] Fix .rel.data.* being erroneously merged into .data on ARM

Fix section pattern code to exclude .rel.data.* sections from being
merged into .data. Otherwise relocations in those sections are lost
in final binary

Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D9108

r311891:
[efi] Build EFI bits with -fPIC on ARM

clang 3.9.0 without -fPIC generates absolute jump table for
switch/case statement which trips boot1.efi and loader.efi
on ARM platform.

Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D9108