powerpc/lib: Add alt patching test of branching past the last instruction

Add a test of the relative branch patching logic in the alternate
section feature fixup code. This tests that if we branch past the last
instruction of the alternate section, the branch is not patched.
That's because the assembler will have created a branch that already
points to the first instruction after the patched section, which is
correct and needs no further patching.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>