* also fix extern variables with initialiaztion values to generate runtime code
* remove the workaround in example/shared_library/mathtest.zig
* introduce the ability for global variables to have Weak and LinkOnce
linkage
* fix `@export` to work for non-functions. this code needs to be
audited though.
* fix comptime ptrcast not keeping bigger alignment
* fix linker warnings when targeting darwin
closes#1903
28bf768883