ext-toolchain: Fix ARCH_SYSROOT detection

ext-toolchain: Fix ARCH_SYSROOT detection

For the detection of the ARCH_SYSROOT_DIR (which contains the C
library variant specific to the compiler flags), we used to pass only
the -march argument instead of the full TARGET_CFLAGS. This was done
because TARGET_CFLAGS contains --sysroot, and we don't want to tell
here the compiler which sysroot to use, because we're specifically
asking the compiler where the *normal* arch sysroot directory is.

Unfortunately, there are some multilib variants that aren't decided
only based on -march, but also on -msoft-float or other compiler
flags. Therefore, we take the opposite approach: pass the full
TARGET_CFLAGS, from which we have stripped the --sysroot option.

For example, this allows a PowerPC CodeSourcery toolchain, on which
we're using the soft-float multilib variant, to work properly as an
external toolchain.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Yann E. MORIN <yann.morin.1998@anciens.enib.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>