Remove 32 bit sparc v7 support

The patch is straighforward:

  - The sparc32 v8 implementations are moved as the generic ones.

  - A configure test is added to check for either __sparc_v8__ or
    __sparc_v9__.

  - The triple names are simplified and sparc implies sparcv8.

The idea is to keep support on sparcv8 architectures that does support
CAS instructions, such as LEON3/LEON4.

Checked on a sparcv9-linux-gnu and sparc64-linux-gnu.

Tested-by: Andreas Larsson <andreas@gaisler.com>