debug/gdb: add handling of 7_0_or_later

We'll need to differentiate gdb 7.0 or later, from gdb prior to 7.0

debug/gdb: add option to use GMP and MPFR

GMP and MPFR are optional for gdb, so offer a config option.

debug/gdb: can't build a static native gdb if using GMP/MPFR

It is currently impossible to reliably build static GMP/MPFR libs,
so we can't build a static native gdb using them, obviously.

debug/gdb: split menuconfig for easier maintenance

cc/gcc: fix use of companion libraries

complibs: hide prompts

scripts: fix dumping the config for companion libraries for target

Companion libraries can be disable for the host, but still used for
the target (eg. binutils or gdb). Dump the version also in this case.

scripts: fix dumping config

libelf has been moved to being a companion library, now.

config: hide entries when in backend mode

This implies hidding entries for:
- binutils libraries
- dmalloc
- duma
- native gdb
- ltrace
- strace
- sstrip

kconfig: silence a warning about undefined env variable

When using an environment variable to set a config entry,
don'twarn if it is not defined. Update doc accordingly.