Merge gdb.arch/system-gcore.exp into gdb.base/gcore.exp

Merge gdb.arch/system-gcore.exp into gdb.base/gcore.exp

The test case gdb.arch/system-gcore.exp is almost an identical copy of
gdb.base/gcore.exp.  The only functional change is the additional
comparison of the "info reg system" command output.

It should be safe to perform this comparison on all architectures.
Thus the patch removes the arch-specific version and adds "info reg
system" to the common gcore test case instead.

Tested on i686-linux and s390x-linux.

gdb/testsuite/Changelog:
2013-05-07  Andreas Arnez  <arnez@linux.vnet.ibm.com>

	* gdb.arch/system-gcore.exp: Remove.
	* gdb.arch/gcore.c: Remove.
	* gdb.base/gcore.exp: Add "info reg system".