x86/retpoline: Make CONFIG_RETPOLINE depend on compiler support

x86/retpoline: Make CONFIG_RETPOLINE depend on compiler support

Since retpoline capable compilers are widely available, make
CONFIG_RETPOLINE hard depend on the compiler capability.

Break the build when CONFIG_RETPOLINE is enabled and the compiler does not
support it. Emit an error message in that case:

 "arch/x86/Makefile:226: *** You are building kernel with non-retpoline
  compiler, please update your compiler..  Stop."

[dwmw: Fail the build with non-retpoline compiler]

Suggested-by: Peter Zijlstra <[email protected]>
Signed-off-by: Zhenzhong Duan <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: David Woodhouse <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Daniel Borkmann <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Konrad Rzeszutek Wilk <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Masahiro Yamada <[email protected]>
Cc: Michal Marek <[email protected]>
Cc: <[email protected]>
Cc: [email protected]
Link: https://lkml.kernel.org/r/cca0cb20-f9e2-4094-840b-fb0f8810cd34@default