OvmfPkg/OvmfPkgX64: Expose DEBUG_TO_MEM as a build option

OvmfPkgX64.dsc has several build options, but some are
missing from the top definition list. This makes it difficult
for users to identify all available configuration flags.

Add DEBUG_TO_MEM to the [Defines] section with a default value of
FALSE to improve visibility.

This change has no functional impact.

Signed-off-by: Richard Lyu <richard.lyu@suse.com>