i486 build defaults to -march=i686 even if -march is specified via CXXFLAGS

I export the CFLAGS, CXXFLAGS and ASFLAGS via a conan profile, which includes some default CXXFLAGS, including the correct `-march`, `-mtune` and other optimizations for an intel Atom prozessor. Unfortunatly, the boost build system is defaulting to i686, even when `-march=silvermont -mtune=silvermont -mfpmath=sse` (Bay Trail Intel Atom) is specified. The reason seems to be in https://github.com/boostorg/build/blob/develop/src/tools/gcc.jam#L1190 . Unfortunatly the list is outdated and is not matching the current one, valid for gcc 8.3+.

I suggest to default to the supplied -march, if specified via the CXXFLAGS and only default to i686, if the information is missing.