powerpc: Tell gcc when we clobber the carry in inline asm

We have several instances of inline assembly code that use the addic
or addic. instructions, but don't include XER in the list of clobbers.
The addic and addic. instructions affect the carry bit, which is in
the XER register.

This adds "xer" to the list of clobbers for those inline asm
statements that use addic or addic. and didn't already have it.

Signed-off-by: Paul Mackerras <paulus@samba.org>