MFC r258625: sdt: add support for solaris/illumos style DTRACE_PROBE macros

Note: in head KDTRACE_HOOKS is defined in opt_global.h and thus there is
no need to explicitly include any header to check that option.
In this branch we have to include opt_kdtrace.h and, so, this commit
carries additional changes to accomodate for that difference.

Sponsored by:	HybridCluster
MFC r260051:
When clearing relocations to __dtrace* symbols, handle both SHT_REL and
SHT_RELA sections properly instead of assuming that the relocation section
is of type SHT_REL.