[PATCH v2] memcpy: use bhs/bls instead of bge/blt [BZ #25620]

[Wilco Dijkstra]

Hi Zhuyan,

As Joseph mentioned the test should not be target specific. If you're thinking of
just adding a test for memcpy for now, then adding it to string/test-memcpy.c would
be fine given it should be part of the basic tests. Existing tests like test-memchr.c already
have special checks for huge inputs. Once we have a working implementation, it would
be easy to copy it to test-memmove.c, test-memset.c etc.

In particular do_test1 shows how to do a mmap with an inaccessible page to avoid
corrupting any memory which could cause the test to pass or fail incorrectly.
The test should run all ifuncs as well, not just one.

Cheers,
Wilco