fuzz: link fuzz programs with `make all` on Linux

Since 5e47215080 (fuzz: add basic fuzz testing target., 2018-10-12), we have compiled object files for the fuzz tests as part of the default 'make all' target. This helps prevent bit-rot in lesser-used parts of the codebase, by making sure that incompatible changes are caught at build time. However, since we never linked the fuzzer executables, this did not protect us from link-time errors. As of 8b9a42bf48 (fuzz: fix fuzz test build rules, 2024-01-19), it's now possible to link the fuzzer executables without using a fuzzing engine and a variety of compiler-specific (and compiler-version-specific) flags, at least on Linux. So let's add a platform-specific option in config.mak.uname to link the executables as part of the default `make all` target. Since linking the fuzzer executables without a fuzzing engine does not require a C++ compiler, we can change the FUZZ_PROGRAMS build rule to use $(CC) by default. This avoids compiler mis-match issues when overriding $(CC) but not $(CXX). When we *do* want to actually link with a fuzzing engine, we can set $(FUZZ_CXX). The build instructions in the CI fuzz-smoke-test job and in the Makefile comment have been updated accordingly. While we're at it, we can consolidate some of the fuzzer build instructions into one location in the Makefile. Suggested-by: Junio C Hamano <gitster@pobox.com> Helped-by: Jeff King <peff@peff.net> Signed-off-by: Josh Steadmon <steadmon@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>