Add documentation for stdarch tests

Author: antoyo

doc/debugging-gcc-lto.md

@@ -0,0 +1,38 @@
+# Debugging
+
+## How to debug GCC LTO
+
+Run do the command with `-v -save-temps` and then extract the `lto1` line from the output and run that under the debugger.
+
+## How to debug stdarch tests that cannot be ran locally
+
+First, run the tests normally:
+
+----
+cd build/build_sysroot/sysroot_src/library/stdarch/
+STDARCH_TEST_EVERYTHING=1 CARGO_TARGET_X86_64_UNKNOWN_LINUX_GNU_RUNNER="sde -future -rtm_mode full --" TARGET=x86_64-unknown-linux-gnu ../../../../../y.sh cargo test
+----
+
+It will show the command it ran, something like this:
+
+----
+  process didn't exit successfully: `sde -future -rtm_mode full -- /home/user/projects/rustc_codegen_gcc/build/build_sysroot/sysroot_src/library/stdarch/target/debug/deps/core_arch-fd2d75f89baae5c6` (signal: 11, SIGSEGV: invalid memory reference)
+----
+
+Then add the `-debug` flag to it:
+
+----
+sde -debug -future -rtm_mode full -- /home/user/projects/rustc_codegen_gcc/build/build_sysroot/sysroot_src/library/stdarch/target/debug/deps/core_arch-fd2d75f89baae5c6
+----
+
+To see the symbols in `gdb`, specify the executable in your command:
+
+----
+gdb /home/user/projects/rustc_codegen_gcc/build/build_sysroot/sysroot_src/library/stdarch/target/debug/deps/core_arch-fd2d75f89baae5c6
+----
+
+and then write the `gdb` command that `sde` tells you to use, something like:
+
+----
+target remote :51299
+----