Fix offsets tool for Android NDK r20

Fix offsets tool for Android NDK r20

Android NDK r20 contains a header file (`ioctl.h`) with this code:

    int ioctl(int __fd, int __request, ...);

    #if !defined(BIONIC_IOCTL_NO_SIGNEDNESS_OVERLOAD)
    /* enable_if(1) just exists to break overloading ties. */
    int ioctl(int __fd, unsigned __request, ...) __overloadable __enable_if(1, "") __RENAME(ioctl);
    #endif

The `_RENAME` macro is defined as `__asm__(x)` and serves the purpose of
replacing an existing symbol with the new one, decorated with the keyword.
Unfortunately, Mono's `offsets-tool` doesn't parse the header properly as
`CppSharp` doesn't understand `__asm__`:

    Processing triple: armv7-none-linux-androideabi
    Error parsing '/Users/rodo/git/xa2/external/mono/mono/metadata/metadata-cross-helpers.c'
    /Users/rodo/android-toolchain/ndk/sysroot/usr/include/bits/ioctl.h(60,5): error: conflicting types for 'ioctl'
    Error parsing '/Users/rodo/git/xa2/external/mono/mono/mini/mini-cross-helpers.c'
    /Users/rodo/android-toolchain/ndk/sysroot/usr/include/bits/ioctl.h(60,5): error: conflicting types for 'ioctl'
    make: *** [/Users/rodo/git/xa2/external/mono/sdks/builds/android-cross-arm-release/armv7-linux-android.h] Error 1

The workaround, implemented in this commit, is to define the
`BIONIC_IOCTL_NO_SIGNEDNESS_OVERLOAD` macro thus removing the "conflicting"
declaration from the picture, which changes nothing from the offset tool's point
of view and lets us process the headers in NDK r20 properly.