Use -fsigned-char to build (#2033)
Use -fsigned-char to build (#2033)

This is found when compiling realm-java with cmake. The different
default sets on different targets cause linking failed with lto enabled.

The problem is, cmake toolchain shipped with google sdk has
 -fsigned-char by default.
See https://github.com/taka-no-me/android-cmake/blob/master/android.toolchain.cmake#L1237

But the default gcc has different signed/unsigned for different
targets, eg.: arm64 by default is unsigned-char.
realm/realm-core,realm/realm-core,realm/realm-core,realm/realm-core,realm/realm-core,realm/realm-core,realm/realm-core