gdb/testsuite/gdb.trace: Deduplicate pcreg/spreg/fpreg.

gdb/testsuite/gdb.trace: Deduplicate pcreg/spreg/fpreg.

These variables were used in many gdb.trace tests.  Keep them in one place,
to reduce work needed for new targets.

gdb/testsuite/ChangeLog:

	* gdb.trace/backtrace.exp: Use global fpreg/spreg definition, add $
	in front.
	* gdb.trace/change-loc.exp: Use global pcreg definition.
	* gdb.trace/collection.exp: Use global pcreg/fpreg/spreg definition.
	* gdb.trace/entry-values.exp: Use global spreg definition, add $
	in front.
	* gdb.trace/mi-trace-frame-collected.exp: Use global pcreg definition.
	* gdb.trace/pending.exp: Likewise.
	* gdb.trace/report.exp: Use global pcreg/fpreg/spreg definition.
	* gdb.trace/trace-break.exp: Likewise.
	* gdb.trace/trace-condition.exp: Use global pcreg definition, add $
	in front.
	* gdb.trace/unavailable.exp: Use global pcreg/fpreg/spreg definition.
	* gdb.trace/while-dyn.exp: Use global fpreg definition, add $
	in front.
	* lib/trace-support.exp: Define fpreg, spreg, pcreg variables.