fuzz: build fuzzers by default on Linux

Please note: this has been rebased onto the current 'master' [436d4e5
(The seventeenth batch, 2024-04-10)] in order to resolve a conflict with
the recently merged bt/fuzz-config-parse series.

Increase our protection against fuzzer bit-rot by making sure we can
link the fuzz test executables on Linux. Patch 1 is a small CI config
improvement to fix compiler feature detection. Patch 2 is the Makefile /
config.mak.uname change to add the executables to `make all` on Linux.

Changes in V2:
* Rebased onto master
* Fixed compiler mismatch issue when we override CC but not CXX
* Consolidated some of the fuzzer Makefile definitions in one location

Change-Id: Ibe2d4c7c60cbf4dd97f44caf1868759f4cfdc785