[phobos] Purity of C Math Functions

[David Simcha]

I'm trying to test out Brad's 64 work and I can't compile Phobos because 
in 64 mode std.math calls some C math functions declared in druntime w/o 
purity, safety, etc. tags.  I now remember that this was one of the few 
areas I meant to come back to when I was cleaning up 64 support in 
Phobos, but I forgot about it.  Is it safe to assume that C 
core.stdc.math functions are @safe pure nothrow?