Enable -fno-altivec, -mno-altivec for PowerPC.

Introduces these negation forms explicitly and uses them to control a new
"altivec" target feature for PowerPC.  This allows avoiding generating
Altivec instructions on processors that support Altivec.

The new test case verifies that the Altivec "lvx" instruction is not
used when -fno-altivec is present on the command line.