update makefile to support ios builds

[tantaman]

so I thought this would be really hard and have been avoiding it for months.

Turns out we just needed to:

1. set the target triple
2. build rust's std from source
3. set the sysroot to the iPhoneOS.sdk

To use:

cr-sqlite> cd core
cr-sqlite/core> export IOS_TARGET=aarch64-apple-ios; make loadable

Now in dist you'll have a crsqlite.dylib for ios

ls -ltrh dist
total 37992
-rw-r--r--  1 tantaman  staff   8.3M Jul  7 21:52 sqlite3-extra.c
-rw-r--r--  1 tantaman  staff   5.1M Jul 10 10:59 libcrsql_bundle-loadable.a
-rwxr-xr-x  1 tantaman  staff   848K Jul 10 10:59 crsqlite.dylib

Caveat

This assumes that we can load dylib extensions into sqlite on iOS. @alanjhughes is investigating this. If we can't then we'll have to build this statically into sqlite which should largely be more of the same changes but for static targets in the Makefile.

[tantaman]

@alanjhughes - you should be able to swap this out to whatever iOS target you need to build for.

I'll make this more configurable so you can specify it from the command-line.
maybe something like EXPORT BUILD_FOR_IOS=your_desired_target