x86/head_64: Use TESTB instead of TESTL in secondary_startup_64_no_verify()

There is no need to use TESTL when checking the least-significant bit
with a TEST instruction. Use TESTB, which is three bytes shorter:

   f6 05 00 00 00 00 01    testb  $0x1,0x0(%rip)

vs:

   f7 05 00 00 00 00 01    testl  $0x1,0x0(%rip)
   00 00 00

for the same effect.

No functional changes intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20231109201032.4439-1-ubizjak@gmail.com