Make linux(4) handle MAP_32BIT.

This unbreaks Mono (mono-devel-4.6.2.7+dfsg-1ubuntu1 from Ubuntu Bionic);
previously would crash on "amd64_is_imm32" assert.

MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D23306