ext-toolchain-wrapper: force hash-style to 'both'

If ext-toolchain-wrapper was built with a gcc that uses hash-style 'gnu' by
default, the resulting binary might be unusable on other systems. The error
in this case is "Floating point exception".
Using hash-style 'both' solves this issue.

Signed-off-by: Patrick Ziegler <patrick.ziegler@fh-kl.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>