SOLARIS: suppress -xarch=amd64 is deprecated warnings

SOLARIS: suppress -xarch=amd64 is deprecated warnings

The -m64 flag to specify 64bit builds was introduced in Sun Studio 10, circa
2005. The old flag -xarch=amd64 was deprecated as of Sun Studio 12, circa 2007.
Ever since Sun Studio 12, the compiler complains with a warning message when
the old -xarch=amd64 flag is given:

    cc: Warning: -xarch=amd64 is deprecated, use -m64 to create 64-bit programs

Update the cflags when building the Solaris kernel module for x86 to use the
modern -m64 under Solaris 11 or later. Since Solaris 11 has been available
since 2010, it is very unlikely a compiler on Solaris 11 would not support the
modern -m64 flag.

Change-Id: Ib13c00f1c69f34ab1905a8dd4a46c90895046f25
Reviewed-on: https://gerrit.openafs.org/12959
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>