scripts/gdb: fix lx-symbols command for arm64 LLVM

lx-symbols assumes that module's .text sections is located at
`module->mem[MOD_TEXT].base` and passes it to add-symbol-file command. 
However, .text section follows after .plt section in modules built by LLVM
toolchain for arm64 target.  Symbol addresses are skewed in GDB.

Fix this issue by using the address of .text section stored in
`module->sect_attrs`.

Link: https://lkml.kernel.org/r/20230801121052.2475183-1-koudai@google.com


Signed-off-by: Koudai Iwahori <koudai@google.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Kieran Bingham <kbingham@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>