Do not use RTLD_DEEPBIND if dlmopen is available

[danog]

This pull request disables usage of RTLD_DEEPBIND if dlmopen is available.

Context:

• Php8.1 with alternative malloc allocators #10670 - The issue this PR fixes, using RTLD_DEEPBIND with custom allocators preloaded via LD_PRELOAD causes segfaults as the main binary is using the custom malloc, while shared libs use the global malloc due to RTLD_DEEPBIND
• Add --enable-rtld-deepbind configure flag, disable it by default except for apache #16779 - My previous PR which disables RTLD_DEEPBIND only on non-apache SAPIs via autoconf at buildtime, which raised some questions when building multiple SAPIs
• Php8.1 with alternative malloc allocators #10670 (comment) - Context as to why RTLD_DEEPBIND was initially added, to avoid symbol conflicts with already loaded libraries when using the embed SAPI for example via apache.

After careful consideration, I believe this is the best approach, after considering the following alternatives:

• Making a PR based on Add zend.dlopen_deepbind php.ini directive #11094, disabling deepbind by default via an ini default except when using the embed SAPI, changing the ini value in php_embed_init; this is still SAPI-dependent behavior, which will still cause the same segfaults if jemalloc is used with the embed SAPI.
• Writing a wrapper for the embed SAPI which dlmopens the real libphp.so within a new namespace (essentially what @dstogov intended with his suggestion, except with RTLD_DEEPBIND the isolation is not propagated, with dlmopen namespaces it is)

I opted for the much cleaner approach of completely disabling RTLD_DEEPBIND if dlmopen with LM_ID_NEWLM is available, leaving to users the resposibility of isolating libphp.so when including it by using dlmopen(LM_ID_NEWLM, "libphp.so", RTLD_LAZY); instead of dlopen("libphp.so", RTLD_LAZY|RTLD_DEEPBIND);.

dlmopen provides full recursive isolation for all symbols both in the opened library, and in libraries opened by that library, avoiding symbol conflict issues even more effectively than RTLD_DEEPBIND, which is not recursive.

Per #10670 (comment), to archieve full real isolation, real-world apache usecases still need to patch apache (libapr) to enable RTLD_DEEPBIND when dlopening libphp.so, as dso_load does not allow passing the RTLD_DEEPBIND flag, so it would not change much for usecases which currently use isolation, just a tweak to the patch that currently adds the RTLD_DEEPBIND flag to libapr to use dlmopen(LM_ID_NEWLM, ...);, instead.

On platforms where GNU extensions aren't available (and dlmopen thus isn't available), RTLD_DEEPBIND is left enabled; if equivalent namespace isolation methods are available on other platforms, they can be added with later pull requests if needed.