fix(ExecSolib): compile trampoline as PIE; abort on ET_EXEC

The trampoline binary embedded in ddtrace.so was produced as ET_EXEC
(non-PIE) by toolchains that don't default to -fPIE (e.g. devtoolset-7
on CentOS 7).  elf_load_trampoline accepted only ET_DYN and used
mmap(NULL) to pick a random load base — an ET_EXEC binary loaded that
way crashes because its absolute virtual addresses no longer match.

Two-pronged fix:
1. libdatadog/spawn_worker/build.rs: add -fPIE/-pie on Linux so the
   trampoline is always ET_DYN, matching the original design intent.
2. solib_bootstrap.c: add a __builtin_trap() guard after the ET_DYN
   check so a mis-built ET_EXEC trampoline aborts loudly instead of
   silently misbehaving.

Fixes "failed to map trampoline" (exit 121) on bookworm-slim for
PHP 8.3-8.5.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: cataphract

ext/solib_bootstrap.c

@@ -709,6 +709,11 @@ static int elf_load_trampoline(const void *src, size_t src_len,
     const Elf64_Ehdr *ehdr = (const Elf64_Ehdr *)src;
 
     if (elf_check_header(ehdr, 1u << ET_DYN, 32) < 0) return -1;
+    // The trampoline must be a PIE (ET_DYN) executable. ET_EXEC cannot be
+    // loaded at a random base address. build.rs enforces -fPIE/-pie; if
+    // something goes wrong in the build and ET_EXEC slips through, abort
+    // loudly rather than silently misbehaving.
+    if (ehdr->e_type != ET_DYN) __builtin_trap();
     if (ehdr->e_phoff + (uint64_t)ehdr->e_phnum * sizeof(Elf64_Phdr) > src_len) return -1;
 
     const Elf64_Phdr *phdrs = (const Elf64_Phdr *)((const char *)src + ehdr->e_phoff);