toolchain: gdb: set -std=gnu17

toolchain: gdb: set -std=gnu17

Fedora 42 updated to GCC15 which now defaults to GNU23 as the default
instead of GNU17[1], and this breaks gdb compilation.

This looks like a readline issue, that was fixed in GDB 16.2 but to
not update toolchain components we can simply set the C standard back
to GNU17.

[1] https://gcc.gnu.org/gcc-15/porting_to.html#c23

Link: https://github.com/openwrt/openwrt/pull/18581
Signed-off-by: Robert Marko <robimarko@gmail.com>