x86/tdx: Share tdx_panic() with the EFI stub

Move the implementation of tdx_panic() into the source file that is
shared with the decompressor and the EFI stub.

Use memcpy() and strnlen() instead of strtomem_pad(), as the latter does
not exist in the early boot code. Avoid fortify instrumentation in the
decompressor/EFI stub by defining __NO_FORTIFY before including
tdx-shared.c

Note that __tdx_hypercall() may call __tdx_hypercall_failed() if the
hypercall returns with an error (while it should never return to begin
with). __tdx_hypercall_failed() calls the decompressor's error()
routine, which prints a message and then loops forever.

When called from the EFI stub, this error() call may attempt to use port
I/O to the default serial port rather than the TDX hypercalls which the
decompressor uses normally to print diagnostics to the console, but this
is fine: given that this situation only occurs after a catastrophic
error, and a subsequent spurious return from tdx_panic(), whether
error() uses port I/O or not is rather moot at that point, as long as it
never returns.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Link: https://patch.msgid.link/20260925075137.44298-6-ardb@kernel.org

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