Assume FreeBSD kernels always report exec events.

Assume FreeBSD kernels always report exec events.

FreeBSD kernels have reported exec events via the PL_FLAG_EXEC flag
since 8.2 release.  The most recent release that did not support this
flag is 7.4 released in November of 2011.

Note that the FreeBSD native target already assumed that PL_FLAG_SCE
and PL_FLAG_SCX were always supported on systems supporting
PT_LWPINFO, but those flags were added at the same time as
PL_FLAG_EXEC.  Building the native target on a system without
PL_FLAG_EXEC would have failed to build before this change already as
a result.

gdb/ChangeLog:

	* fbsd-nat.c (fbsd_nat_target::wait): Always check for
	PL_FLAG_EXEC.
	(fbsd_nat_target::insert_exec_catchpoint)
	(fbsd_nat_target::remove_exec_catchpoint): Always define.
	* fbsd-nat.h (fbsd_nat_target::insert_exec_catchpoint)
	(fbsd_nat_target::remove_exec_catchpoint): Always declare.