powerpc/kexec: Don't use .machine ppc64 in trampoline_64.S

As best as I can tell the ".machine" directive in trampoline_64.S is no
longer, or never was, necessary.

It was added in commit 0d976313 ("powerpc: Add purgatory for
kexec_file_load() implementation."), which created the file based on
the kexec-tools purgatory. It may be/have-been necessary in the
kexec-tools version, but we have a completely different build system,
and we already pass the desired CPU flags, eg:

  gcc ... -m64 -Wl,-a64 -mabi=elfv2 -Wa,-maltivec -Wa,-mpower4 -Wa,-many
  ... arch/powerpc/purgatory/trampoline_64.S

So drop the ".machine" directive and rely on the assembler flags.
Reported-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Segher Boessenkool <segher@kernel.crashing.org>
Link: https://lore.kernel.org/r/20210315034159.315675-1-mpe@ellerman.id.au

arch/powerpc/purgatory/trampoline_64.S

@@ -12,7 +12,6 @@
 #include <asm/asm-compat.h>
 #include <asm/crashdump-ppc64.h>
 
-	.machine ppc64
 	.balign 256
 	.globl purgatory_start
 purgatory_start: