[gdb/contrib] Handle STRIP_ARGS_{STRIP,KEEP}_DEBUG in cc-with-tweaks.sh

Handle new environment variable STRIP_ARGS_STRIP_DEBUG, defaulting to
--strip-debug in gdb/contrib/cc-with-tweaks.sh, such that we can easily
reproduce the PR29277 assert using:
...
$ export STRIP_ARGS_STRIP_DEBUG=--strip-all
$ make check RUNTESTFLAGS="gdb.base/jit-reader.exp \
    --target_board cc-with-gnu-debuglink"
...

For completeness sake and to avoid confusion about which of the two used strip
invocations the passed args apply to, likewise add STRIP_ARGS_KEEP_DEBUG,
defaulting to --only-keep-debug.

Script checked with shellcheck, no new warnings added.

Tested on x86_64-linux.