package/guile: dont't use -Os for CS 2014.05 ARM toolchain

With this toolchain, guile tigger an assembler error with -Os which is
probably caused by the binutils version being used:

arm-none-linux-gnueabi-ld --version
GNU ld (Sourcery CodeBench Lite 2014.05-29) 2.24.51.20140217

arm-none-linux-gnueabi-gcc --version
arm-none-linux-gnueabi-gcc (Sourcery CodeBench Lite 2014.05-29) 4.8.3 20140320 (prerelease)

Nowaday, even with a Buildroot internal toolchain using the oldest
Binutils GCC version (2.25.1 and 4.8.6), guile build fine with -Os.

So, force -O2 when this toolchain is used and BR2_OPTIMIZE_S is set.

Fixes:
http://autobuild.buildroot.net/results/976/976b3477407e64c8c2fc2309ed952d0083903a19



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>