Fix building android on x86

In 70360cb11, the ASM variable was moved to the x86-common file
even though the android build file didn't include neither
platform-arch.mk nor platform-x86-common.mk.

Instead of explicitly declaring ASM here, include platform-arch.mk
and remove setting of the flags that platform-arch.mk sets.

This is inspired by and based on a patch by Licai Guo.
Merge pull request #421 from mstorsjo/android-x86-build

Fix building android on x86