internal/bytealg: fix Separator length check for Index/ppc64le
internal/bytealg: fix Separator length check for Index/ppc64le

Modified condition in the ASM implementation of indexbody to
determine if separator length crosses 16 bytes to BGT from BGE
to avoid incorrectly crossing a page.

Also fixed IndexString to invoke indexbodyp9 when on the POWER9
platform

Change-Id: I0602a797cc75287990eea1972e9e473744f6f5a9
Reviewed-on: https://go-review.googlesource.com/c/go/+/356849
Run-TryBot: Lynn Boger <b27445fe6fb259c37b8e0a4ce56e22e2413754bf@linux.vnet.ibm.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Lynn Boger <b27445fe6fb259c37b8e0a4ce56e22e2413754bf@linux.vnet.ibm.com>
Trust: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>

golang/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,cloudflare/go,golang/go,golang/go,nawawi/go,CAFxX/go,nawawi/go,CAFxX/go,CAFxX/go,nawawi/go,CAFxX/go,cloudflare/go,CAFxX/go,cloudflare/go,cloudflare/go,nawawi/go,CAFxX/go,nawawi/go,cloudflare/go,cloudflare/go,golang/go,nawawi/go,nawawi/go,cloudflare/go,CAFxX/go,golang/go,golang/go,golang/go