Turn on partial LFS for posix like platforms.

Turn on partial LFS for posix like platforms.

When running b2 inside a qemu mixed mode environment (32 inside 64)
it errors with some system calls not properly translating from 64 to 32.
This works around the problem by making the 32 bit b2 build use 64 bit
file system calls in libc.

Potentially fix for:
* https://github.com/bfgroup/b2/issues/49
* https://github.com/conan-io/conan-center-index/issues/6059
* https://github.com/boostorg/build/issues/663

Change-Id: Ibe6f41f1dd67adb15b6c2551229df96357d887b5
Signed-off-by: DongHun Kwak <dh0128.kwak@samsung.com>