toolchain/external: fix building the wrapper on MIPS

Some crazy folks use MIPS machines as build machines. ;-)

On MIPS, the only acceptable hash-style is 'sysv', because the MIPS ABI
defines that the GOT ordering to be the same as the symbols ordering,
while GNU hash requires symbols to be sorted by their hash.

Looking at binutils' code, it seems that only MIPS suffers from that
limitation.

Currently, we force the toolchain wrapper to be linked with both hash
styles, which breaks on MIPS.

So, fix that by singling out MIPS, and use sysv in that case, and both
otherwise.

Reported-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Tested-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>