Support all cross-supported targets.

Adds Android, iOS, Linux/ARM, Linux/ARM/musl, and Windows. Bare-metal
thumb targets are listed, because they are supported by cross, but they
aren't enabled by default as they don't support std and will likely not
but used by most projects.

Because of a bug in musl on the `*-musleabi*` targets, the hello.c test
case has been reduced to use unistd and write() instead of printf().

.travis.yml

@@ -18,9 +18,30 @@ matrix:
   # TODO These are all the build jobs. Adjust as necessary. Comment out what you
   # don't need
   include:
+    # Android
+    - env: TARGET=aarch64-linux-android DISABLE_TESTS=1
+    - env: TARGET=arm-linux-androideabi DISABLE_TESTS=1
+    - env: TARGET=armv7-linux-androideabi DISABLE_TESTS=1
+    - env: TARGET=i686-linux-android DISABLE_TESTS=1
+
+    # iOS
+    - env: TARGET=aarch64-apple-ios DISABLE_TESTS=1
+      os: osx
+    - env: TARGET=armv7-apple-ios DISABLE_TESTS=1
+      os: osx
+    - env: TARGET=armv7s-apple-ios DISABLE_TESTS=1
+      os: osx
+    - env: TARGET=i386-apple-ios DISABLE_TESTS=1
+      os: osx
+    - env: TARGET=x86_64-apple-ios DISABLE_TESTS=1
+      os: osx
+
     # Linux
     - env: TARGET=aarch64-unknown-linux-gnu
+    - env: TARGET=arm-unknown-linux-gnueabi
+    - env: TARGET=arm-unknown-linux-musleabi
     - env: TARGET=armv7-unknown-linux-gnueabihf
+    - env: TARGET=armv7-unknown-linux-musleabihf
     - env: TARGET=i686-unknown-linux-gnu
     - env: TARGET=i686-unknown-linux-musl
     - env: TARGET=mips-unknown-linux-gnu
@@ -45,6 +66,16 @@ matrix:
     - env: TARGET=x86_64-unknown-freebsd DISABLE_TESTS=1
     - env: TARGET=x86_64-unknown-netbsd DISABLE_TESTS=1
 
+    # Windows
+    - env: TARGET=x86_64-pc-windows-gnu
+
+    # Bare metal
+    # These targets don't support std and as such are likely not suitable for
+    # most crates.
+    # - env: TARGET=thumbv6m-none-eabi
+    # - env: TARGET=thumbv7em-none-eabi
+    # - env: TARGET=thumbv7em-none-eabihf
+    # - env: TARGET=thumbv7m-none-eabi
 
     # Testing other channels
     - env: TARGET=x86_64-unknown-linux-gnu

hello.c

@@ -1,5 +1,5 @@
-#include <stdio.h>
+#include <unistd.h>
 
 void hello() {
-  printf("Hello, world!\n");
+    write(0, "Hello, world!\n", 14);
 }