libcpu: riscv: declare external symbols inside

libcpu/risc-v/common64/mmu.c uses the external
symbol "__bss_end" (this symbol generally is
defined in the link script file) and depends
on the extern declaration of this symbol in
the bsp's "board.h". This is not a problem in
implementation, but it is not a good habit.

If this extern symbol is used locally, just
declare it locally should be better.

In this way, there will be no dependency of
the kernel core on the bsp header file. And
there will be no special requirements for the
bsp board code, such as the need to make
extern declarations for these "__bss_end"
symbols in "board.h".

This patch currently only explicitly externly
declares "__bss_end" inside libcpu itself.
The code of "#include <board.h>" is kept, because
the modification involved is too large, it is
necessary to clean up the bsp involved before
cleaning this include code line uniformly.

Signed-off-by: Chen Wang <unicorn_wang@outlook.com>

libcpu/risc-v/common64/mmu.c

@@ -648,6 +648,8 @@ void rt_hw_mmu_setup(rt_aspace_t aspace, struct mem_desc *mdesc, int desc_nr)
 
 #define SATP_BASE ((rt_ubase_t)SATP_MODE << SATP_MODE_OFFSET)
 
+extern unsigned int __bss_end;
+
 /**
  * @brief Early memory setup function for hardware initialization.
  *