tests/tcg: compile system emulation tests as freestanding

tests/tcg: compile system emulation tests as freestanding

System emulation tests do not run in a hosted environment, since they
do not link with libc.  They should only use freestanding headers
(float.h, limits.h, stdarg.h, stddef.h, stdbool.h, stdint.h,
stdalign.h, stdnoreturn.h) and should be compiled with -ffreestanding
in order to use the compiler implementation of those headers
rather than the one in libc.

Some tests are using inttypes.h instead of stdint.h, so fix that.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>