gdb: fix full gdb build for MIPS musl

gdb: fix full gdb build for MIPS musl

Currently building full gdb for MIPS musl fails because it's trying to
include <sgidefs.h> which is provided by glibc and uClibc, but not by
musl.

However, the kernel headers provide <asm/sgidefs.h> which has the same
definitions, so we can use that one instead.

Backporting a patch that has been sent upstream. Taken from here:

https://sourceware.org/bugzilla/show_bug.cgi?id=21070

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>