Filter out '-std=gnu99' when running the assembler. This causes an error

Filter out '-std=gnu99' when running the assembler. This causes an error
when building MIPS that looks like this:

libc/string/mips/memcpy.S:156:1: pasting "memcpy" and ":" does not give
a valid preprocessing token
make[1]: *** [libc/string/mips/memcpy.os] Error 1

Even if this does not cause an error on other architectures, this option
should not be having an effect on preprocessed assembly code.