x86_64: Fix memory access size for locore EOI

Newer QEMU (5.1) hangs / timeouts on a number of tests on x86_64.  In
debugging the issue this is related to a fix in QEMU 5.1 that
validates memory region access.  QEMU has the APIC region only allowing
1 to 4 byte access.  64-bit access is treated as an error.

Change the APIC EOI access in locore.S back to just doing a 32-bit
access.

Fixes #	28453

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>