[Pal/Linux-SGX] Detect signal stack overflow in "enclave_entry.S"

[boryspoplawski]

Description of the changes

Creating a frame with CPU context on signal stack when handling an exception was done without any bound checks. Malicious host could generate any amount of signals and cause these frames to overflow the stack. This commit fixes it by adding bound checks, effectively disallowing more than ~9 nested exceptions (which should be more than enough in normal case).

How to test this PR?

Try running in Gramine with SGX:

#define _GNU_SOURCE
#include <err.h>
#include <sched.h>

int main(void) {
     while (1) {
         if (sched_yield() < 0) {
             err(1, "sched_yield");
         }
     }
}

and concurrently timeout 4 bash -c 'while true; do kill -SIGCONT pid_of_above_program; done'. This will cause weird behavior on the current master (most likely a crash) and with this PR the process will hang in newly added FAIL_LOOP

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This change is 

[mkow]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: all files reviewed, all discussions resolved, not enough approvals from maintainers (1 more required), not enough approvals from different teams (1 more required, approved so far: ITL)

[dimakuv]

Reviewed all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved