Start selectively disabling a few kernel build warnings for clang, since

there are some places in the kernel where fixing them is too disruptive,
or where there is a false positive.

In this case, disable -Wconstant-conversion for two aic7xxx-related
files, as they get the following warning on i386 (and possibly on other
32-bit arches):

sys/dev/aic7xxx/ahc_pci.c:112:10: warning: implicit conversion from 'long long' to 'bus_addr_t' (aka 'unsigned int') changes value from 549755813887 to 4294967295 [-Wconstant-conversion]
                                   ? 0x7FFFFFFFFFLL
                                   ~~^~~~~~~~~~~~~~

This is a false positive, since the code only passes the 0x7FFFFFFFFFLL
argument, if sizeof(bus_addr_t) is larger than 4 (e.g. on 64 bit arches,
or when PAE is enabled on i386).  The code could be refactored to do
compile-time checks, but that is more disruptive.

MFC after:	1 week

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