tools/firmware: Build firmware as -ffreestanding

firmware should always have been -ffreestanding, as it doesn't execute in the
host environment.  -ffreestanding implies -fno-builtin, so replace the option.

inttypes.h isn't a freestanding header, but the 32bitbios_support.c only wants
the stdint.h types so switch to the more appropriate include.

This removes the build time dependency on a 32bit libc just to compile the
hvmloader and friends.

Update README and the TravisCI configuration.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Ian Jackson <iwj@xenproject.org>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>

README

@@ -62,9 +62,6 @@ provided by your OS distributor:
     * GNU bison and GNU flex
     * GNU gettext
     * ACPI ASL compiler (iasl)
-    * Libc multiarch package (e.g. libc6-dev-i386 / glibc-devel.i686).
-      Required when building on a 64-bit platform to build
-      32-bit components which are enabled on a default build.
 
 In addition to the above there are a number of optional build
 prerequisites. Omitting these will cause the related features to be