powerpc/ftrace: Refactor ftrace_{regs_}caller

ftrace_caller() and frace_regs_caller() have now a lot in common.

Refactor them using GAS macros.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/9d7df9e4fc98a86051489f61d3c9bc67f92f7e27.1645099283.git.christophe.leroy@csgroup.eu