Align stack correctly when initializing a context

In `my_context_spawn`, assuming the allocated stack pointer is 16-bye
aligned (as it will be from malloc, etc), align the stack correctly
before calling the target function of the context.

For i686 platform, The Linux ABI requires that the stack is aligned on a
16-byte boundary at the time that a call is made, and the compiler
attempts to maintain that alignment by adjusting the stack as required
to meet the alignment requirements for data, and any subsequent calls.

This is particularly important when compiling for i386 platform with
-msse2, The compiler can emit code to use SSE2 instructions like
`movaps`, which will actually fault on unalgned access. This causes the
mariadb test suite to fail with the following error, for example:

```
main.non_blocking_api                    w4 [ fail ]
        Test ended at 2023-08-18 00:33:40

CURRENT_TEST: main.non_blocking_api
mysqltest got signal 11
read_command_buf (0x89a2a88): connect (con_nonblock,localhost,root,,test)

conn->name (0x89c7208):

Attempting backtrace...
stack_bottom = 0x0 thread_stack 0x10000
mysys/stacktrace.c:213(my_print_stacktrace)[0x80a7348]
bits/stdio2.h:97(fprintf)[0x80615f2]
addr2line: '': No such file
[0xf7ee8560]
??:0(BIO_f_buffer)[0xf7bc1ca1]
??:0(BIO_vsnprintf)[0xf7bc2aa1]
??:0(BIO_snprintf)[0xf7bc2af4]
??:0(ASN1_UTCTIME_adj)[0xf7be0875]
??:0(X509_time_adj_ex)[0xf7c062dd]
??:0(X509_time_adj)[0xf7c06312]
??:0(X509_cmp_time)[0xf7c06516]
??:0(X509_cmp_time)[0xf7c06720]
??:0(X509_verify_cert)[0xf7c08641]
??:0(ssl_verify_cert_chain)[0xf7d172e1]
??:0(ssl3_get_server_certificate)[0xf7cefc96]
??:0(ssl3_connect)[0xf7cf520b]
??:0(ssl23_connect)[0xf7cfee42]
secure/openssl.c:487(ma_tls_connect)[0x808ac22]
libmariadb/ma_tls.c:83(ma_pvio_tls_connect)[0x807d82d]
libmariadb/ma_pvio.c:531(ma_pvio_start_ssl)[0x807d553]
auth/my_auth.c:318(send_client_reply_packet)[0x808f5db]
auth/my_auth.c:94(native_password_auth_client)[0x808f84e]
auth/my_auth.c:620(run_plugin_auth)[0x808fb5c]
libmariadb/mariadb_lib.c:1665(mthd_my_real_connect)[0x807a121]
libmariadb/mariadb_async.c:332(mysql_real_connect_start_internal)[0x808b3d9]
libmariadb/ma_context.c:440(my_context_spawn)[0x808ed69]
libmariadb/mariadb_lib.c:3927(mariadb_get_socket)[0x8078c6f]
tests/nonblock-wrappers.h:96(wait_for_mysql(st_mysql*, int))[0x806117e]
```

In this case, the eventual crash is in openssl code, but this is a
conseqeunce of a `movaps` instruction in `BIO_f_buffer` acting on an
unaligned stack location, which is in turn a result of the unaligned
stack from `my_context_spawn`. Any use of SSE2 instructions requiring
alignment will fail in the same way

libmariadb/ma_context.c

@@ -450,6 +450,11 @@ my_context_spawn(struct my_context *c, void (*f)(void *), void *d)
      */
      ".cfi_escape 0x07, 8\n\t"
 #endif
+     /*
+      * we'll push the arg on the stack. Skipping the first 12 bytes will mean the
+      * stack is re-aligned on a 16-byte address when we jump to "f".
+      */
+     "sub $12, %%esp\n\t"
      /* Push the parameter on the stack. */
      "pushl %[d]\n\t"
      "movl %%ebp, 4(%[save])\n\t"