remote.c, QCatchSyscalls: Build std::string instead of unique_xmalloc_ptr

remote.c, QCatchSyscalls: Build std::string instead of unique_xmalloc_ptr

Simplify the code a little bit using std::string + string_appendf.

gdb/ChangeLog:
2017-10-30  Pedro Alves  <palves@redhat.com>
	    Simon Marchi <simon.marchi@ericsson.com>

	* remote.c (remote_set_syscall_catchpoint): Build a std::string
	instead of a gdb::unique_xmalloc_ptr, using string_appendf.