OvmfPkg/Sec: Fix 64bit SEC build failure

Original code breaks a single assembly code to multiple lines.
But, when VS CL.exe preprocesses the FixedPcdGet32() macro
invocation to the replacement text, it loses '\', and causes
NASM to fail.

Changing the multiple lines to one line to resolve the build failure.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>