doc: note GCC >= 14 requirement for native riscv64 builds

V8's deps/v8/src/base/cpu.cc unconditionally includes <riscv_vector.h>
and uses __attribute__((target("arch=+v"))) for runtime RVV detection
when V8_HOST_ARCH_RISCV64 is defined (native builds only). GCC 13 does
not support the RISC-V target attribute and its riscv_vector.h cannot
be included without -march=rv64gcv, so native riscv64 builds require
GCC >= 14 or Clang >= 19.

Add a footnote to the riscv64 row in the platform list table to
document this. Cross-compilation from x64 is not affected.

Refs: nodejs/build#4099
Signed-off-by: Jamie Magee <jamie.magee@gmail.com>

Author: JamieMagee

BUILDING.md

@@ -115,7 +115,7 @@ platforms. This is true regardless of entries in the table below.
 | GNU/Linux        | ppc64le >=power9 | kernel >= 4.18[^1], glibc >= 2.28 | Tier 2       | e.g. Ubuntu 20.04, RHEL 8            |
 | GNU/Linux        | s390x >=z14      | kernel >= 4.18[^1], glibc >= 2.28 | Tier 2       | e.g. RHEL 8                          |
 | GNU/Linux        | loong64          | kernel >= 5.19, glibc >= 2.36     | Experimental |                                      |
-| GNU/Linux        | riscv64          | kernel >= 5.19, glibc >= 2.36     | Experimental |                                      |
+| GNU/Linux        | riscv64          | kernel >= 5.19, glibc >= 2.36     | Experimental | GCC >= 14 or Clang >= 19 for native builds[^7] |
 | Windows          | x64              | >= Windows 10/Server 2016         | Tier 1       | [^2],[^3]                            |
 | Windows          | arm64            | >= Windows 10                     | Tier 2       |                                      |
 | macOS            | x64              | >= 13.5                           | Tier 1       | For notes about compilation see [^4] |
@@ -148,6 +148,13 @@ platforms. This is true regardless of entries in the table below.
 [^4]: Our macOS Binaries are compiled with 13.5 as a target. Xcode 16 is
     required to compile.
 
+[^7]: Native riscv64 builds need GCC >= 14 or Clang >= 19 because V8
+    includes `<riscv_vector.h>` and uses `target("arch=+v")` in
+    `deps/v8/src/base/cpu.cc`. GCC 13's `riscv_vector.h` errors out without
+    `-march=rv64gcv` and doesn't support the `target` attribute at all.
+    Cross-compilation from x64 is unaffected (the code is behind
+    `V8_HOST_ARCH_RISCV64`).
+
 <!--lint enable final-definition-->
 
 ### Supported toolchains