sys/sys: Check for ASSEMBLER in elf_common.h

When preprocessing assembly files with clang or gcc the ASSEMBLER
macro is defined. Check for this as an alternative to LOCORE in
elf_common.h so it can be included by .S files.

Reviewed by: imp
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D42224

(cherry picked from commit f39010b1393d471f1c59195acde5d95cdb66847c)