core: override the user's locale while setting HOSTARCH

At least with French user's locale HOSTARCH is empty since
'Target' is not present in gcc output.

gcc -v 2>&1
Utilisation des specs internes.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/5.3.1/lto-wrapper
Cible : x86_64-redhat-linux

Override the user's local with LC_ALL=C.

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>