build: Add -mfloat-abi=softfp on Android

It seems that Android requires `-mfloat-abi=softfp -mfpu=neon` in the
compilation flags in order to use the NEON instructions, even if the
configure time check passes with `-mfpu=neon` alone.

[ebassi/graphene#12]
Merge pull request #20 from ebassi/issues/12

build: Add -mfloat-abi=softfp on Android