Use CCASFLAGS when assembling .S sources

This allows overriding whether all of CFLAGS should be passed
when assembling.
Don't pass all MSVC specific cflags to gas-preprocessor and armasm

This avoids having to disambiguate between assembler parameters,
preprocessor parameters and MSVC parameters within gas-preprocessor.

In particular, this avoids creating a stray dependency file named
'-.d' while building. Future versions of gas-preprocessor might
require this in order to build at all.
Merge pull request #832 from mstorsjo/asflags

Don't pass all MSVC specific cflags to gas-preprocessor and armasm