powerpc/bpf: Rename PPC_BL_ABS() to PPC_BL()

PPC_BL_ABS() is just doing a relative branch with link. The name
suggests that it is for branching to an absolute address, which is
incorrect. Rename the macro to a more appropriate PPC_BL().

Signed-off-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/f0e57b6c7a6ee40dba645535b70da46f46e8af5e.1644834730.git.naveen.n.rao@linux.vnet.ibm.com