Self build toolchains for RISC-V lacking correct libc

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The problem about lacking __libc_init_array entry due to the newlib in riscv32-gnu-toolchain changed HAVE_INITFINI_ARRAY to _HAVE_INITFINI_ARRAY according to this commit:

commit 437c5c5085ff30b4a4960b2b53d06728c788361d
Author: Mike Frysinger <vapier@gentoo.org>
Date:   Mon Jan 17 22:20:20 2022 -0500

    newlib: internalize HAVE_INITFINI_ARRAY
   
    This define is only used by newlib internally, so stop exporting it
    as HAVE_INITFINI_ARRAY since this can conflict with defines packages
    use themselves.
   
    We don't really need to add _ to HAVE_INIT_FINI too since it isn't
    exported in newlib.h, but might as well be consistent here.
   
    We can't (easily) add this to newlib_cflags like HAVE_INIT_FINI is
    because this is based on a compile-time test in the top configure,
    not on plain shell code in configure.host.  We'd have to replicate
    the test in every subdir in order to have it passed down.

This led to the build of vx_syscalls.c lack of entry of __libc_init_array and __libc_fini_array. I fixed this by this change:

diff --git a/kernel/src/vx_syscalls.c b/kernel/src/vx_syscalls.c
index 6ff9fbb..ad0777b 100644
--- a/kernel/src/vx_syscalls.c
+++ b/kernel/src/vx_syscalls.c
@@ -65,7 +65,7 @@ void __init_tls(void) {
   memset(__thread_self + (size_t)__tbss_offset, 0, (size_t)__tbss_size);
 }
 
-#ifdef HAVE_INITFINI_ARRAY
+#ifdef _HAVE_INITFINI_ARRAY
 
 // These magic symbols are provided by the linker.
 extern void (*__preinit_array_start []) (void) __attribute__((weak));
@@ -96,7 +96,7 @@ void __libc_init_array (void) {
 }
 #endif
 
-#ifdef HAVE_INITFINI_ARRAY
+#ifdef _HAVE_INITFINI_ARRAY
 extern void (*__fini_array_start []) (void) __attribute__((weak));
 extern void (*__fini_array_end []) (void) __attribute__((weak));

The remaining puzzle is how to bootstrap build this libc32 myself.