gdbserver: Fix C++ build errors in tracepoint.c

gdbserver: Fix C++ build errors in tracepoint.c

These were introduced by 1cda1512689aabb36588a01370002632a0c8e560
and a13c46966d308297a1273e35ccc807a3912d573d .  One is a simple
missing cast, the other is const usage on global function pointers
exported from IPA: in C++, consts are static, and thus won't be
exported from the DSO (the build error was because of non-applicable
visibility("default")).

gdb/gdbserver/ChangeLog:

	* tracepoint.c (gdb_collect_ptr): Remove const qualifier.
	(get_raw_reg_ptr): Likewise.
	(get_trace_state_variable_value_ptr): Likewise.
	(set_trace_state_variable_value_ptr): Likewise.
	(initialize_tracepoint): Cast alloc_jump_pad_buffer result to
	char *.