Fix freestanding no malloc guards

[patkenneally]

Make EIGEN_NO_MALLOC mean something in release builds

Two independent fixes to freestanding Eigen, found while investigating four -Wreturn-type warnings in a riscv32-elf-g++ -Os -DNDEBUG build.

1. Delete the EIGEN_NO_MALLOC allocation entry points instead of asserting
   Memory.h guarded six allocators with bodies that contained only eigen_assert(false && "heap allocation is forbidden"). Under NDEBUG, eigen_plain_assert expands to nothing (Macros.h:1004-1017), so those became empty functions returning void* - undefined behaviour if ever called.

This is NDEBUG-specific rather than target-specific: it reproduces on a host with g++-13 -DEIGEN_FREESTANDING=1 -DNDEBUG and disappears without -DNDEBUG.

The six (handmade_aligned_malloc/free/realloc,conditional_aligned_malloc/free/realloc<false>) are now deleted declarations under EIGEN_NO_MALLOC. No body means no warning and no fall-through, and any real use becomes a compile error naming the caller.

Safe to delete: in freestanding EIGEN_DEFAULT_ALIGN_BYTES == 0, so aligned_malloc takes the plain-malloc branch and never reaches handmade_aligned_*. Running nm on compiled products confirms none of these symbols are referenced.

2. Define alloca as a macro so Eigen detects it

Freestanding/portable_stdlib.h declared alloca as an extern "C" function, but Eigen selects its stack-allocation path with #if ... || (defined alloca)
(Memory.h:613) — a macro test, which a declaration never satisfies.

Consequence: EIGEN_ALLOCA stayed undefined on the freestanding target, so ei_declare_aligned_stack_constructed_variable fell to the #else branch at Memory.h:794, which calls aligned_malloc unconditionally, at any size, with no EIGEN_STACK_ALLOCATION_LIMIT check. Hosted builds have alloca and keep small temporaries on the stack, which is why this never showed up there.

Replaced with #define alloca __builtin_alloca, which GCC emits inline on every target including RISC-V. Verified EIGEN_ALLOCA is now defined under riscv32-elf-g++ -ffreestanding.

EIGEN_STACK_ALLOCATION_LIMIT is deliberately left at its default: it is overloaded between that ternary and a static assert capping fixed-size stack objects (DenseStorage.h:33), so tuning it for one breaks the other.

Scope

Both changes are inside #ifdef EIGEN_NO_MALLOC / the freestanding branch. The #else paths keep the original definitions, so hosted and normal Eigen builds are untouched.

Testing

-fsyntax-only on <Eigen/Dense> clean in three configurations: RV32 freestanding -Os -DNDEBUG, RV32 freestanding -Og, and hosted -O2 plain Eigen.