Protect against overlapping with the 32-bit kernel

This patch adds a guard in the linker script to abort the build when the
bootloader code overflows the 12k limit. Indeed, 32-bit Linux with LPAE
enabled puts its initial page tables between 0x80003000 and 0x80007fff.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>