Wrong architecture flags with GCC on x32

Hi, if I build Boost with GCC 8.2 under architecture x32 (i.e., Intel 64 bits used with 32 bits long and pointers, see https://en.wikipedia.org/wiki/X32_ABI) the build fails, because the build system incorrectly puts `-m32` in the compilation flags. The right native mode would be instead `-mx32`, or, even better, no `-m` flag at all and letting the compiler automatically pick the native mode.

In Debian I have to comment line https://github.com/boostorg/build/blob/develop/src/tools/gcc.jam#L440 to build correctly the package. Could you please support this use case?