Cleanup of elf_machdep.c

Fix some style(9) violations.

This also changes the name of the machine-dependent sysctl kern.debug_kld to
debug.kld_reloc, and changes its type from int to bool. This is acceptable
since we are not currently concerned with preserving the RISC-V ABI.

Reviewed by: markj, kp
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D21772