[PPC64] r2 register shouldn't be clobbered in asm

### HHVM Version
3.24.7

### Operating System and Version
Ubuntu 18.10 using gcc7 (https://gcc.gnu.org/gcc-7/changes.html)

### Standalone code, or other way to reproduce the problem
Building

### Expected result
Build success

### Actual result
hphp/util/portability.h:203:25: error: PIC register clobbered by ‘r2’ in ‘asm’