[roll] Roll fuchsia [kernel][x86] Use KERNEL_ASPACE_SIZE to build PTEs

In start.S, use KERNEL_ASPACE_SIZE to calculate the offset into
`pdp_high` for the kernel address space.

This allows us to modify the constants and affect the kernel address
space size correctly. This assumes that the kernel address space size is
less than or equal to 512GB on x86.

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/709503
Original-Revision: 65c72891a67fadcd1dcf816253e9fce17b249d88
GitOrigin-RevId: 893bbf8d8c8ec8a203634db815e25b9518c30aa1
Change-Id: I8e64b964a9b258711a32913690496c54c4286ec8

stem

diff --git a/stem b/stem
index 749d1b7..4544be5 100644
--- a/stem
+++ b/stem
@@ -10,7 +10,7 @@
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              remote="https://fuchsia.googlesource.com/fuchsia"
              gerrithost="https://fuchsia-review.googlesource.com"
-             revision="b4452af210e1db01b0684118ae3e14c1f8eb959a"/>
+             revision="65c72891a67fadcd1dcf816253e9fce17b249d88"/>
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   <hooks>
     <hook name="install-environment"