powerpc/vmlinux.lds: Add an explicit symbol for the SRWX boundary

[ Upstream commit b150a4d1

 ]

Currently __init_begin is used as the boundary for strict RWX between
executable/read-only text and data, and non-executable (after boot) code
and data.

But that's a little subtle, so add an explicit symbol to document that
the SRWX boundary lies there, and add a comment making it clear that
__init_begin must also begin there.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220916131422.318752-2-mpe@ellerman.id.au
Stable-dep-of: 111bcb37

 ("powerpc/64s/radix: Fix RWX mapping with relocated kernel")
Signed-off-by: Sasha Levin <sashal@kernel.org>