Merge #20560: fuzz: Link all targets once

fa13e1b0c5 build: Add option --enable-danger-fuzz-link-all (MarcoFalke)
44444ba759 fuzz: Link all targets once (MarcoFalke)

Pull request description:

  Currently the linker is invoked more than 150 times when compiling with `--enable-fuzz`. This is problematic for several reasons:

  * It wastes disk space north of 20 GB, as all libraries and sanitizers are linked more than 150 times
  * It wastes CPU time, as the link step can practically not be cached (similar to ccache for object files)
  * It makes it a blocker to compile the fuzz tests by default for non-fuzz builds #19388, for the aforementioned reasons
  * The build file is several thousand lines of code, without doing anything meaningful except listing each fuzz target in a highly verbose manner
  * It makes writing new fuzz tests unnecessarily hard, as build system knowledge is required; Compare that to boost unit tests, which can be added by simply editing an existing cpp file
  * It encourages fuzz tests that re-use the `buffer` or assume the `buffer` to be concatenations of seeds, which increases complexity of seeds and complexity for the fuzz engine to explore; Thus reducing the effectiveness of the affected fuzz targets

  Fixes #20088

ACKs for top commit:
  practicalswift:
    Tested ACK fa13e1b0c5
  sipa:
    ACK fa13e1b0c5. Reviewed the code changes, and tested the 3 different test_runner.py modes (run once, merge, generate). I also tested building with the new --enable-danger-fuzz-link-all

Tree-SHA512: 962ab33269ebd51810924c51266ecc62edd6ddf2fcd9a8c359ed906766f58c3f73c223f8d3cc49f2c60f0053f65e8bdd86ce9c19e673f8c2b3cd676e913f2642

src/test/fuzz/script_sigcache.cpp

@@ -16,7 +16,7 @@
 #include <string>
 #include <vector>
 
-void initialize()
+void initialize_script_sigcache()
 {
     static const ECCVerifyHandle ecc_verify_handle;
     ECC_Start();
@@ -24,7 +24,7 @@ void initialize()
     InitSignatureCache();
 }
 
-void test_one_input(const std::vector<uint8_t>& buffer)
+FUZZ_TARGET_INIT(script_sigcache, initialize_script_sigcache)
 {
     FuzzedDataProvider fuzzed_data_provider(buffer.data(), buffer.size());