sparc: VIS mnemonics

Currently, if VIS is enabled by configure, it will also be enabled at
run-time regardless of its support in the hardware. Thus, masking VIS
usage as it is done in vis.h by constructing binary instructions is
pointless. Using normal VIS mnemonics in inline assembly allows to take
advantage of automatic register allocation, gets rid of register
variables, which are unsupported by suncc for SPARC, and improves code
readability.

Signed-off-by: Diego Biurrun <diego@biurrun.de>