chore: Treate the prof=true backtrace method as an enum (#2)

When --enable-prof is set during configure, detection of a suitable
backtrace method is performed stopping with the first supported
solution. This can be augmented by explicitly enabling or disabling
detection of specific methods. To better model this in a bazel context,
the //settings/flags:enable_prof is extended to have enumerable values
where the backtrace method can be explicitly declared.

tools/cc.bzl

@@ -47,4 +47,7 @@ COPTS = ["-D_REENTRANT"] + select({
     ],
     "@platforms//os:macos": ["-DPIC"],
     "//conditions:default": [],
+}) + select({
+    "//settings/flags:prof_gcc": ["-fno-omit-frame-pointer"],
+    "//conditions:default": [],
 })