powerpc/vdso: Inconditionally use CFUNC macro

During merge of commit 4e991e3c ("powerpc: add CFUNC assembly
label annotation") a fallback version of CFUNC macro was added at
the last minute, so it can be used inconditionally.

Fixes: 4e991e3c ("powerpc: add CFUNC assembly label annotation")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/0fa863f2f69b2ca4094ae066fcf1430fb31110c9.1724313540.git.christophe.leroy@csgroup.eu