x86: Expand test-memset.c and bench-memset.c

No bug. This commit adds tests cases and benchmarks for page cross and
for memset to the end of the page without crossing. As well in
test-memset.c this commit adds sentinel on start/end of tstbuf to test
for overwrites

Signed-off-by: Noah Goldstein <goldstein.w.n@gmail.com>