powerpc/ftrace: Use STK_GOT in ftrace_mprofile.S

Instead of open coding offset value 24, use STK_GOT when
accessing got register in stack.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/9042bb30fa972056715fe5b6598a7c8049681293.1645099283.git.christophe.leroy@csgroup.eu