sys-libs/glibc: rework sparc target selection #323445 #361779

sys-libs/glibc: rework sparc target selection #323445 #361779

The current code assumes all targets are at least sparc v9.  This breaks
people trying to build for older/embedded targets like sparc v8.  Only use
specific targets when the user has set -mcpu.  This does mean we will be
using a lower target for some users (who don't have -mcpu set), but that's
more in line with what we want.  Similarly, do not assume that because we
are using a 64-bit kernel we always want a 64-bit userland or newer cpu.

We also drop filtering of -mvis flags (we haven't been filtering the newer
options like -mvis2 or -mvis3) as it doesn't seem to be an issue.

We also drop the filtering of -Wa,-xarch and -Wa,-A flags.  We want to let
the user select their own, or just rely on the -mcpu setting.  This might
mean for some users they get slightly slower builds if they haven't set an
explicit -mcpu flag, but that's also what we want.