GDB fixes

[ziuziakowska]

This PR fixes:

• Without libexpat, GDB is not able to remote connect to QEMU.
• Since switching away from Crosstool-NG, GDB erroneously builds without ELF support(!). Caused by:

configure:16552: checking for ELF support in BFD
configure:16571: gcc -o conftest -I../../gdb/../include -I../bfd -I../../gdb/../bfd -g -O2  -L../bfd -L../libiberty -L../zlib   conftest.c -lbfd -liberty -lz  -lncursesw -lm -ldl  >&5
In file included from conftest.c:189:
../../gdb/../bfd/elf-bfd.h: In function 'bfd_section_is_ctf':
../../gdb/../bfd/elf-bfd.h:3069:10: error: implicit declaration of function 'strncmp' [-Wimplicit-function-declaration]
 3069 |   return strncmp (name, ".ctf", 4) == 0 && (name[4] == 0 || name[4] == '.');
      |          ^~~~~~~

This also closes #93 as we can now verify that the original patch works.

[jwnrt]

LGTM, thanks