Implement flexible target specification

Removes all target-specific knowledge from rustc. Some targets have changed
during this, but none of these should be very visible outside of
cross-compilation. The changes make our targets more consistent.

iX86-unknown-linux-gnu is now only available as i686-unknown-linux-gnu. We
used to accept any value of X greater than 1. i686 was released in 1995, and
should encompass the bare nimimum of what Rust supports on x86 CPUs.

The only two windows targets are now i686-pc-windows-gnu and
x86_64-pc-windows-gnu.

The iOS target has been renamed from arm-apple-ios to arm-apple-darwin.

A complete list of the targets we accept now:

arm-apple-darwin
arm-linux-androideabi
arm-unknown-linux-gnueabi
arm-unknown-linux-gnueabihf

i686-apple-darwin
i686-pc-windows-gnu
i686-unknown-freebsd
i686-unknown-linux-gnu

mips-unknown-linux-gnu
mipsel-unknown-linux-gnu

x86_64-apple-darwin
x86_64-unknown-freebsd
x86_64-unknown-linux-gnu
x86_64-pc-windows-gnu

Closes #16093

[breaking-change]

Conflicts:
	src/librustc/back/link.rs
	src/librustc/middle/trans/cabi_x86.rs
	src/librustc_back/archive.rs

Author: emberian

configure

@@ -300,37 +300,37 @@ case $CFG_OSTYPE in
         ;;
 
     MINGW32*)
-        CFG_OSTYPE=pc-mingw32
+        CFG_OSTYPE=pc-windows-gnu
         ;;
 
     MINGW64*)
         # msys2, MSYSTEM=MINGW64
-        CFG_OSTYPE=w64-mingw32
+        CFG_OSTYPE=pc-windows-gnu
         ;;
 
 # Thad's Cygwin identifers below
 
 #   Vista 32 bit
     CYGWIN_NT-6.0)
-        CFG_OSTYPE=pc-mingw32
+        CFG_OSTYPE=pc-windows-gnu
         CFG_CPUTYPE=i686
         ;;
 
 #   Vista 64 bit
     CYGWIN_NT-6.0-WOW64)
-        CFG_OSTYPE=w64-mingw32
+        CFG_OSTYPE=pc-windows-gnu
         CFG_CPUTYPE=x86_64
         ;;
 
 #   Win 7 32 bit
     CYGWIN_NT-6.1)
-        CFG_OSTYPE=pc-mingw32
+        CFG_OSTYPE=pc-windows-gnu
         CFG_CPUTYPE=i686
         ;;
 
 #   Win 7 64 bit
     CYGWIN_NT-6.1-WOW64)
-        CFG_OSTYPE=w64-mingw32
+        CFG_OSTYPE=pc-windows-gnu
         CFG_CPUTYPE=x86_64
         ;;