* 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