Update toolchains for latest SDK; add CI - #61

On Linux, GDB tests that got deadlocked (for example
run_gdb_break_continue_thread_test on ARM) would cause command_tester.py
to hang indefinitely because the GDB and sel_ldr processes continued to
live after the main test process (the child of command_tester) was
killed. Make the main test process respond to SIGINT by killing its
children.

illwieckz's latest toolchain build
https://github.com/DaemonEngine/saigo-nacl-sdk/releases/tag/v21.0-20260805
adds GDB and renames some binutils binaries. Update scons and run.py
accordingly. Also delete the code for finding GDB in a 2nd variant of
the NaCl toolchain.

But disable 2 tests on ARM that don't work without GDB speaking XML.

In 22bd955 I updated the expected
return code for run_ntdll_intercept_test but neglected to do the same
for run_ntdll_fallback_test.

The TestGettingRegisterSnapshotInSyscallContextSwitch part suspended the
thread inside the syscall on all 10000 iterations on most runs. But when
the thread did suspend outside it, the test seemingly always failed.
The test for the stack pointer value was too strict.

Partial revert of commit 8ebf53f. Turns
out this is still broken on x86-32.

I used the Azure Pipelines job skeleton from
#39. But CMake stuff
is replaced with Scons since that is the currently functional build
system.

All 64-bit platforms are running most tests. ARM Linux has the NaCl
mode disabled due to SDK defects, so it only runs a few tests. x86
32-bit platforms build everything but don't run tests.

Co-authored-by: Thomas Debesse <dev@illwieckz.net>