fix(aspacem): raise amd64 managed address ceiling to 1 TiB

snmalloc-rs as a global allocator aborts under Callgrind because its
startup mmap of a flat 256 GiB MAP_NORESERVE range returns EINVAL. On
amd64-linux the address-space manager capped the managed range at 128
GiB, placing vStart (the client/Valgrind split) at ~64 GiB, so the
largest floating client hole was ~62 GiB and any larger reservation was
vetoed by VG_(am_get_advisory).

Raise aspacem_maxAddr to 1 TiB (vStart ~512 GiB, ~512 GiB client hole),
scoped to VGP_amd64_linux so other 64-bit ports keep 128 GiB (some arm64
kernels expose only a 39-bit user VA). No effect on Callgrind placement
semantics; the fork ships only the 64-bit Callgrind tool.

Refs COD-3073

Author: not-matthias

coregrind/m_aspacemgr/aspacemgr-linux.c

@@ -1862,7 +1862,17 @@ Addr VG_(am_startup) ( Addr sp_at_startup )
                     sp_at_startup );
 
 #  if VG_WORDSIZE == 8
+#    if defined(VGP_amd64_linux)
+     // Startup inserts a one-page reservation at vStart (the midpoint of
+     // [minAddr, maxAddr]), splitting the initial free space. A floating
+     // client mmap searches upward from cStart, so the largest contiguous
+     // hole it can get runs up to that split, i.e. ~half of maxAddr. 128G
+     // left only a ~64G client hole, rejecting multi-hundred-GB lazily-backed
+     // reservations. 1T gives a ~512G hole, well within amd64's 47-bit VA.
+     aspacem_maxAddr = (Addr)0x10000000000ULL - 1; // 1T
+#    else
      aspacem_maxAddr = (Addr)0x2000000000ULL - 1; // 128G
+#    endif
 #    ifdef ENABLE_INNER
      { Addr cse = VG_PGROUNDDN( sp_at_startup ) - 1;
        if (aspacem_maxAddr > cse)