qemu-common: Move tcg_enabled() etc. to sysemu/tcg.h

qemu-common: Move tcg_enabled() etc. to sysemu/tcg.h

Other accelerators have their own headers: sysemu/hax.h, sysemu/hvf.h,
sysemu/kvm.h, sysemu/whpx.h.  Only tcg_enabled() & friends sit in
qemu-common.h.  This necessitates inclusion of qemu-common.h into
headers, which is against the rules spelled out in qemu-common.h's
file comment.

Move tcg_enabled() & friends into their own header sysemu/tcg.h, and
adjust #include directives.

Cc: Richard Henderson <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>
Message-Id: <20190523143508[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
[Rebased with conflicts resolved automatically, except for
accel/tcg/tcg-all.c]