Linux: Keep termios ioctl constants strictly internal

Linux: Keep termios ioctl constants strictly internal

Undefine TCGETS, TCGETS2, and related ioctl constants in the installed
headers.  Extract the correct constants (using the kernel type
definitions) automatically from the UAPI headers.  The kernel
constants are available under KERNEL_* names during the glibc build,
computed using assembler constant extraction mechanism.

Alpha may have to use TCGETS instead of TCGETS2 because TCTGETS2
became available in Linux 4.20 only.  Introduce ARCH_TCGETS to make
this choice explict.

To support emulation on powerpc, glibc versions of the termios
constants are added to the emulation code in internal-ioctl.h.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>