The -MD option (for enabling a dynamically linked crt) gets interpreted
as a cpp option for generating dependency information (into a file named
'-.d', when preprocessing to a pipe). We shouldn't be passing
any and all C compiler flags to armasm (which is a plain assembler,
only with cpp bolted on via gas-preprocessor), but these are the
main conflicting ones.

Signed-off-by: Martin Storsjö <martin@martin.st>