[build] F: Use --allow-multiple-definition for libnvx_crt0 link

End-to-end validation surfaced a hard linker error in the original
--whole-archive approach: libnvx_crt0.a and libposix.a both bundle the
`sys` crate (Rust monomorphizes the FFI `__kcall_*` symbols into both
archives), so --whole-archive on libnvx_crt0 produces "multiple
definition of __kcall_*" errors against libposix.

Switch to:

  -Wl,--allow-multiple-definition
  -Wl,--start-group
    $(wildcard $(SYSROOT_PATH)/lib/libnvx_crt0.a)
    libposix.a libc.a [libm.a]
  -Wl,--end-group

With libnvx_crt0.a inside the start-group and ahead of libposix.a, the
linker resolves the entry-point `_start` from libnvx_crt0 (strong T
symbol) over newlib's weak `_start` fallback. The remaining
__kcall_* duplicates from the two `sys` monomorphizations are
byte-identical (same Rust source, same release profile, same compiler
options) and silently coalesced by --allow-multiple-definition.

The $(wildcard ...) trick preserves backward compatibility: on
pre-PR-11b sysroots libnvx_crt0.a is absent, the wildcard expands to
empty, and the link line collapses to the original behavior.

Validated by building + running the test ELF via nanvixd.exe against
a locally-built nanvix-dev sysroot that includes PR-11b.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: esaurez

src/Makefile

@@ -59,11 +59,16 @@ export LIBC := $(NANVIX_TOOLCHAIN)/i686-nanvix/lib/libc.a
 # ELFs built against the new sysroot fall back to newlib's weak default
 # `_start` and hang at startup with no output.
 export LIBNVX_CRT0 := $(wildcard $(NANVIX_SYSROOT)/lib/libnvx_crt0.a)
-export LIBRARIES := -Wl,--whole-archive $(LIBNVX_CRT0) -Wl,--no-whole-archive \
-                    -Wl,--start-group $(LIBPOSIX) $(LIBC) -Wl,--end-group
+# --allow-multiple-definition: libnvx_crt0 and libposix both bundle the
+# `sys` crate (Rust FFI symbols `__kcall_*` are exported by both).  Both
+# copies are byte-identical (same source, same release profile), so it
+# is safe to let the linker take whichever it sees first.  See the
+# `libnvx_crt0-duplicates-sys-symbols` upstream issue for the proper fix.
+export LIBRARIES := -Wl,--allow-multiple-definition \
+                    -Wl,--start-group $(LIBNVX_CRT0) $(LIBPOSIX) $(LIBC) -Wl,--end-group
 export LIBCXX := $(NANVIX_TOOLCHAIN)/i686-nanvix/lib/libstdc++.a
-export LIBRARIES_CXX := -Wl,--whole-archive $(LIBNVX_CRT0) -Wl,--no-whole-archive \
-                        -Wl,--start-group $(LIBPOSIX) $(LIBC) $(LIBCXX) -Wl,--end-group
+export LIBRARIES_CXX := -Wl,--allow-multiple-definition \
+                        -Wl,--start-group $(LIBNVX_CRT0) $(LIBPOSIX) $(LIBC) $(LIBCXX) -Wl,--end-group
 
 # Output directory for compiled binaries.
 export BINARIES_DIR ?= $(CURDIR)/../build