Fix gdb.base/corefile2.exp test case for ppc64le

It turns out that the recently added gdb.base/corefile2.exp test won't
run on ppc64le linux.  The test case fails the internal checks which
ensure that a mmap'd region can be placed within the statically
allocated regions buf_rw[] and buf_ro[].

ppc64le linux apparently has 64k pages, which is much larger than
the 24k regions originally allocated for buf_rw[] and buf_ro[].

This patch increases the size of each region to 256 KiB.

Tested on either rawhide or Fedora 32 for these architectures: x86_64,
x86_64/-m32, ppc64le, aarch64, and s390x.

gdb/testsuite/ChangeLog:

	* gdb.base/coremaker2.c (buf_rw): Increase size to 256 KiB.
	(C5_24k): Delete.
	(C5_8k, C5_64k, C5_256k): New macros.
	(buf_ro): Allocate 256 KiB of initialized data.