linux-record: Simplify with record_mem_at_reg()

linux-record: Simplify with record_mem_at_reg()

The function record_linux_system_call() often records a memory area
whose address is contained in a register.  So far this required two
function calls: one for fetching the register value, and another one for
recording the memory area.  These two function calls are now merged into
a new local helper function, and all occurrences are adjusted.  This
reduces the source code and makes it more readable.

gdb/ChangeLog:

	* linux-record.c (record_mem_at_reg): New helper function.
	(record_linux_system_call): Exploit new helper function where
	applicable.