runtime: fix memhash64 on big endian systems
runtime: fix memhash64 on big endian systems

Fixes #21677.

Change-Id: I869dee5f43df5d87d86922681726297e3024c562
Reviewed-on: https://go-review.googlesource.com/59810
Run-TryBot: Michael Munday <19411bdeea90f848b8e02e9a1c63a7ce7cc699ea@ibm.com>
Reviewed-by: Martin Möhrmann <9c82b83f348009b39856043bb9ed3ef41e171aeb@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

akutz/go,cloudflare/go,codestation/go,golang/go,josharian/go,akutz/go,polyverse-security/go,josharian/go,polyverse-security/go,golang/go,josharian/go,CAFxX/go,cloudflare/go,CAFxX/go,nawawi/go,polyverse-security/go,golang/go,codestation/go,nawawi/go,akutz/go,josharian/go,Cofyc/go,akutz/go,Cofyc/go,polyverse-security/go,christopher-henderson/Go,CAFxX/go,golang/go,Cofyc/go,polyverse-security/go,CAFxX/go,cloudflare/go,nawawi/go,akutz/go,codestation/go,golang/go,Cofyc/go,akutz/go,akutz/go,nawawi/go,nawawi/go,codestation/go,golang/go,codestation/go,golang/go,Cofyc/go,polyverse-security/go,josharian/go,CAFxX/go,josharian/go,akutz/go,golang/go,Cofyc/go,polyverse-security/go,nawawi/go,nawawi/go,cloudflare/go,cloudflare/go,josharian/go,cloudflare/go,cloudflare/go,codestation/go,cloudflare/go,polyverse-security/go,codestation/go,CAFxX/go,nawawi/go,codestation/go,Cofyc/go,CAFxX/go,CAFxX/go,Cofyc/go,josharian/go