gdb: add fix for gdb 7.12 and gdb 8.0 build on noMMU platforms

This adds a patch to gdb 7.12 and gdb 8.x, which fixes the build on
noMMU platforms. It is not needed for older versions of gdb, since
it's related to the switch of gdb to C++ in the 7.12 release.

Fixes:

../nat/linux-ptrace.c: In function 'int linux_fork_to_function(gdb_byte*, int (*)(void*))':
../nat/linux-ptrace.c:273:29: error: invalid conversion from 'void*' to 'gdb_byte* {aka unsigned char*}' [-fpermissive]
       child_stack = xmalloc (STACK_SIZE * 4);

The patch has already been merged upstream, as of commit
ffce45d2243e5f52f411e314fc4e1a69f431a81f, and will therefore be part
of future gdb releases.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>