Go frontend patch committed: Update builtin function attributes

[Ian Lance Taylor]

This patch updates the attributes for the relatively few builtin
functions defined by the Go frontend to the current ones in
builtins.def. This fixes PR 118746. Bootstrapped and ran Go
testsuite on x86_64-pc-linux-gnu. Committed to mainline.

Ian

PR go/118746
* go-gcc.cc (class Gcc_backend): Define builtin_cold,
builtin_leaf, builtin_nonnull. Alphabetize constants.
(Gcc_backend::Gcc_backend): Update attributes for builtin
functions to match builtins.def.
(Gcc_backend::define_builtin): Split out attribute setting into
set_attribtues.
(Gcc_backend::set_attribtues): New method split out of
define_builtin. Support new flag values.