x86/startup_64: Simplify calculation of initial page table address

Determining the address of the initial page table to program into CR3
involves:
- taking the physical address
- adding the SME encryption mask

On the primary entry path, the code is mapped using a 1:1 virtual to
physical translation, so the physical address can be taken directly
using a RIP-relative LEA instruction.

On the secondary entry path, the address can be obtained by taking the
offset from the virtual kernel base (__START_kernel_map) and adding the
physical kernel base.

This is implemented in a slightly confusing way, so clean this up.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Tested-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20240227151907.387873-14-ardb+git@google.com