MFC r309400:
Fix for endless recursion in the ACPI GPE handler during boot.

When handling a GPE ACPI interrupt object the EcSpaceHandler()
function can be called which checks the EC_EVENT_SCI bit and then
recurse on the EcGpeQueryHandler() function. If there are multiple GPE
events pending the EC_EVENT_SCI bit will be set at the next call to
EcSpaceHandler() causing it to recurse again via the
EcGpeQueryHandler() function. This leads to a slow never ending
recursion during boot which prevents proper system startup, because
the EC_EVENT_SCI bit never gets cleared in this scenario.

The behaviour is reproducible with the ALASKA AMI in combination with
a newer Skylake based mainboard in the following way:

Enter BIOS and adjust the clock one hour forward. Save and exit the
BIOS. System fails to boot due to the above mentioned bug in
EcGpeQueryHandler() which was observed recursing multiple times.

This patch adds a simple recursion guard to the EcGpeQueryHandler()
function and also also adds logic to detect if new GPE events occurred
during the execution of EcGpeQueryHandler() and then loop on this
function instead of recursing.

Reviewed by: jhb