This commit adds the AEX-Notify flows inside the enclave.

The stage-1 signal handler is augmented as follows when AEX-Notify is
enabled: manually restore SSA[0] context, invoke the EDECCSSA
instruction instead of EEXIT (to go from SSA[1] to SSA[0] without
exiting the enclave) and finally jump to SSA[0].GPRSGX.RIP to resume
enclave execution (it will resume in stage-2 signal handler).

The stage-2 signal handler is augmented as follows: set bit 0 of
SSA[0].GPRSGX.AEXNOTIFY (so that AEX-Notify starts working again for
this thread), then apply AEX-Notify mitigations and finally restore
regular enclave execution.

This commit does not add any real AEX-Notify mitigations. Instead, we
count the number of AEX events reported inside the SGX enclave and print
this number on enclave termination (if log level is at least "warning").

Note that current implementation of AEX-Notify does not use the
checkpoint mechanism described in the official AEX-Notify whitepaper.
That checkpoint mechanism allows to coalesce multiple AEX events
that occur during the execution of mitigations. This saves some CPU
cycles and some signal-handling stack space, but we leave implementing
this optimization as future work.

Signed-off-by: Dmitrii Kuvaiskii <[email protected]>

Fixed GDB issue. Fixed a SIGSEGV data race on thread termination
(ERESUME morphs into EENTER but then performs EEXIT). Added AEXNOTIFY
envvar to LibOS regression tests (but only to a subset from
`manifest.template`, simply because changing all manifest template files
would be a huge git diff).

Signed-off-by: Dmitrii Kuvaiskii <[email protected]>

Fixed EDMM issue. Turned out to be a case of too many nested signal
handlers inside Gramine's SGX PAL, which overflowed the SGX enclave
signal stack.

Signed-off-by: Dmitrii Kuvaiskii <[email protected]>

This commit adds conditional AEX-Notify enablement to all Gramine tests.

Run tests e.g. like this (on a machine that supports AEX-Notify both in
hardware and in Linux kernel):

    $ EDMM=1 AEXNOTIFY=1 SGX=1 gramine-test pytest

Signed-off-by: Dmitrii Kuvaiskii <[email protected]>