Android: Add ImportC definitions

[eakmyr]

This PR fixes broken ImportC on Android targets. Tested on sqlite3, zlib, lwIP.

[thewilsonator]

please file this upstream to DMD and once that is accepted with review (e.g. __builtin_memset and friend have recently beed added to DMD, though maybe that was for APPLE I forget, and #define alignof(x) sizeof(x)` looks suspect) cherrypick this back to LDC if you need to.

[eakmyr]

__builtin_memset and friend

there's only one for APPLE, and __builtin_memset**_chk** and same with friends

#define alignof(x) **sizeof**(x)

true thanks. changed sizeof(x) to _Alignof(x)

opened the PR at DMD with these changes. let's see..

[eakmyr]

Removed obsolete builtins. These changes rely on #5106 and #5107

[thewilsonator]

DMD PR is dlang/dmd#22814

[thewilsonator]

Merged dependant PRs

[eakmyr]

This PR's changes are now consistent with the DMD PR

[eakmyr]

__alignof__ is not Android-specific. The problem is that some C libs use __alignof__ with two trailing underscores, so I opened another PR #5108 that adds it below the __alignof define.