Fetch all registers before writing the core register notes.

Fetch all registers before writing the core register notes.

Without this, not all registers were present in the core generated by
gcore.  For example, running 'gcore' on a program without examining
the vector registers (SSE or AVX) would store all the vector registers
as zeros because they were not pulled into the regcache.  Running
'info vector' before 'gcore' would store the correct values in the
core since it populated the regcache.  For Linux processes, a similar
operation is achieved by having the thread iterator callback invoke
target_fetch_registers on each thread before its corresponding
register notes are dumped.

gdb/ChangeLog:

	* fbsd-tdep.c (fbsd_make_corefile_notes): Fetch all target registers
	before writing core register notes.