mingw.zig: fix logic to add crt sources
mingw.zig: fix logic to add crt sources

The current version of code uses isARM to check if we are compiling to any arm target then checks the target bit width to either add the 32-bit sources or 64-bit source.  However, isARM only returns true for 32-bit targets, and isAARCH64 is for the 64-bit targets.

I also replaced the unreachable with a @panic when we receive an unsupported arch because this code is reachable and should turn into an error.

andrewrk/zig,andrewrk/zig