SIGILL in main.non_blocking_api test on aarch64 with Branch Target Identification (BTI)

Full description and patch in https://github.com/mariadb-corporation/mariadb-connector-c/pull/276 and there is CONC-774 for mariadb-connector-c itself.

Text from github issue reproduced below:

Fixes `SIGILL` in MariaDB Server `main.non_blocking_api` test case on aarch64 processors supporting Branch Target Identification (BTI).

Branch Target Identification is a new aarch64 feature that prevents jumping to arbitrary code locations that are not properly marked as branch targets.

The `-mbranch-protection=standard` flag to GCC will enable code generation using BTI. This is backwards compatible as the added instructions are no-ops on older processors.

One example of where both `-mbranch-protection=standard` is enabled and hardware supports it is with Amazon Linux 2023 and Graviton 4.

The symptom is the following mtr `main.non_blocking_api` failure:

  mysqltest got signal 4

  read_command_buf (0xaaaae6ae27a8): connect

  (con_nonblock,localhost,root,,test)

  conn->name (0xaaaae6b0c0b8):

  Attempting backtrace...

  stack_bottom = 0x0 thread_stack 0x3c000

  /usr/bin/mariadb-test(my_print_stacktrace+0x48)[0xaaaab6934088]

  bits/stdio2.h:79(signal_handler)[0xaaaab68f41a0]

  addr2line: 'linux-vdso.so.1': No such file

  linux-vdso.so.1(__kernel_rt_sigreturn+0x0)[0xffffa22bc830]

  /usr/bin/mariadb-test(my_context_spawn+0x74)[0xaaaab6914d84]

With GDB giving the information that SIGILL was on the perfectly legal 'mov' instruction:

  Program received signal SIGILL, Illegal instruction.

  0x0000aaaaaab44d84 in my_context_spawn (c=0xaaaaab07ada8, f=0xfffff7fed240, d=0x0) at

  mariadb1011-10.11.11-1.amzn2023.0.1.aarch64/libmariadb/libmariadb/ma_context.c:674

  674       __asm__ __volatile__

  (gdb) x/i$pc

  => 0xaaaaaab44d84 <my_context_spawn(my_context*, void (*)(void*),

  void*)+116>: mov      w0, #0x1                        // #1

The call sequence to get to this point is as follows:

  Breakpoint 1, my_context_spawn (c=0xaaaaab07ada8, f=0xaaaaaab38024

  <mysql_real_connect_start_internal(void*)>, d=0xffffffffc648) at

  libmariadb/ma_context.c:661

  661       register void *stack asm("x13") = c->stack_top;

  (gdb) c

  Continuing.

  Breakpoint 2, my_context_yield (c=0xaaaaab07ada8) at

  libmariadb/ma_context.c:843

  843       register const uint64_t *save asm("x19") = &c->save[0];

  Continuing from here leads to the SIGILL above.

The branch that ends up causing the SIGILL is the `br x11` instruction in `my_context_yield()`. This is due to the branch being to a location that is not marked as a branch target.

The fix is to ensure that these locations in the `my_context` aarch64 assembler are marked with `bti j` instructions so that the locations that will be `br`ed to are valid to be `br`ed to.

Since older compilers don't know about the instruction, we need an `ifdef` to define a string that has the raw opcode in it.

Fixes: d2285fb8304c82fd87de483845d1c12b06927479 (introduced in v3.3.12)

—
*NOTE*: This will need to make its way into MariaDB Server as well in order for the fix to appear there.