The minimum supported version of GCC to build the x86 kernel was bumped
to GCC 8.1 in
a3e8fe814ad1 ("x86/build: Raise the minimum GCC version to 8.1").
'-mindirect-branch' and '-mindirect-branch-register' were first supported in
GCC 8.1, so there is no need to call cc-option to inquire if it is supported.