x86/sev: Don't emit BSS_DECRYPTED section unless it is in use

x86/sev: Don't emit BSS_DECRYPTED section unless it is in use

The BSS_DECRYPTED section that gets emitted into .bss will be empty if
CONFIG_AMD_MEM_ENCRYPT is not defined. However, due to the fact that it
is injected into .bss rather than emitted as a separate section, the
2 MiB alignment that it specifies is still taken into account
unconditionally, pushing .bss out to the next 2 MiB boundary, leaving
a gap that is never freed.

So only emit a non-empty BSS_DECRYPTED section if it is going to be
used.  In that case, it would still be nice to free the padding, but
that is left for later.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://patch.msgid.link/20260108092526.28586-23-ardb@kernel.org