[libc] F: Weaken _do_start so libnvx_crt0.a can override it cleanly

newlib/libc/sys/nanvix/crt0.S

@@ -13,7 +13,33 @@
  * Exported Symbols                                                           *
  *============================================================================*/
 
-.globl _do_start
+/*
+ * `_do_start` is the kernel-entry trampoline that lands here via IRET from
+ * a freshly-created process trap frame.  EDX holds `argp` and ECX holds
+ * `envp`.  This stub pushes both arguments and calls `_start`.
+ *
+ * Declared `.weak` so that consumers linking Nanvix's `libnvx_crt0.a` (which
+ * provides a stronger SSE-aligned `_do_start` that satisfies the i386 SysV
+ * ABI's 16-byte stack-alignment requirement at the CALL site) cleanly
+ * override this fallback at link time.  Bare-newlib consumers (no
+ * `libnvx_crt0.a` in the link) still get the original behavior because the
+ * weak symbol is the only definition the linker sees.
+ *
+ * Archive-extraction caveat: standard ld will NOT pull a member out of an
+ * archive solely to override an already-satisfied weak symbol.  For the
+ * override to take effect, `libnvx_crt0.a`'s `_do_start` member must be
+ * extracted via some other reference (a co-archive symbol the link
+ * actually needs) or via `-Wl,--whole-archive`.  The current Nanvix
+ * link lines satisfy this prerequisite; a future refactor that strips
+ * `libnvx_crt0.a` down to a single-symbol member must re-add a hook
+ * (an extra reference, or `--whole-archive`) or the broken-for-SSE
+ * fallback below will silently win again.
+ *
+ * This change eliminates the duplicate-strong-symbol clash between
+ * `crt0.o` and `libnvx_crt0.a` that previously forced every Nanvix guest
+ * binary's link line to use `-Wl,--allow-multiple-definition`.
+ */
+.weak _do_start
 
 /*============================================================================*
  * Text Section                                                               *