[testsuite][AArch64] Port gdb.trace

[testsuite][AArch64] Port gdb.trace

This patch adds support for AArch64 to the gdb.trace testsuite.

Note that it does not add support for testing fast tracepoint as it
isn't supported.  Therefore the test cases with inline assembly are not
ported in this patch, as we do not know what the conditions for
inserting a fast tracepoint on AArch64 would be.

gdb/testsuite/ChangeLog:

	* gdb.trace/backtrace.exp: Set registers for aarch64 target.
	* gdb.trace/collection.exp: Likewise.
	* gdb.trace/mi-trace-frame-collected.exp: Likewise.
	* gdb.trace/mi-trace-unavailable.exp: Likewise.
	* gdb.trace/report.exp: Likewise.
	* gdb.trace/trace-break.exp: Likewise.
	* gdb.trace/unavailable.exp: Likewise.
	* gdb.trace/while-dyn.exp: Likewise.