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.

 reporter_name="unknown"

 reporter_url="http://iphonelinux.org/index.php/Glibc_toolchain_using_crosstool-NG"

 reporter_comment="An (experimental) toolchain targeting the Apple's iPhone."