debug/gdb: fix runtime sysroot

This patch sets the runtime sysroot to fix the following GDB warning:
"Unable to find dynamic linker breakpoint function.
GDB will be unable to debug shared library initializers
and track explicitly loaded dynamic code."

The sysroot can later be changed within gdb with the `set sysroot`
command if necessary.

Signed-off-by: "Benoît THÉBAUDEAU" <benoit.thebaudeau@advansee.com>
(transplanted from 7690196856ce4a4700d1887cc3adf80acc627c52)

# SPARC specific config options

config ARCH_sparc

    select ARCH_SUPPORTS_32

    select ARCH_SUPPORTS_64

    select ARCH_DEFAULT_32

    select ARCH_USE_MMU

    select ARCH_SUPPORT_CPU

    select ARCH_SUPPORT_TUNE

    help

      The SUN SPARC architecture, as defined by:

        32 bit: http://www.sparc.org/standards/V8.pdf

        64 bit: http://www.sparc.org/standards/SPARCV9.pdf