apache: fix download URL
apr-util: fix download URL
cryptsetup: fix URL of a mirror
elfutils: remove dead links
epdfview: remove dead upstream mirror, add BLFS one instead
gimp: provide working mirrors
json-c: provide working mirrors
kbd: remove dead mirror
startup-notification: download url on yocto mirror changed
midori: provide working mirrors
mysql: add working mirror
svn: remove broken dl URL
add tool to check download URLs
gdb: make it possible to build gdbserver with WANT_GDBSERVER=1
gdb: support all target archs supported by musl

this allows to debug binaries for other archs via

    qemu-microblaze -g 6000 ./a.out
    gdb ./a.out -ex "target remote :6000"

note that gdb only supports DWARF2 debug info, so it may be required
to pass -gdwarf-2 when compiling your binaries with unpatched
GCC >= 4.8, which defaults to DWARF4.