ld: Change the name of section .user_mmu_data to user_mmu_tables

The arch/x86/CMakeLists.txt build scripts names five sections that are
generated from .bin files. Two of them are named the same as the .bin
file, and the other three are named inconsistently.

To be consistent, we will rename the three that are named inconistenly
to align with the two that are named as the .bin file.

Being consistent simplifies the system and fosters code-reuse.

This patch renames user_mmu_tables.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>