riscv/crt0: Avoid clobbering the caller's stack in _trap

Currently the trap frame will overwrite previously saved registers, so the
GDB backtrace is somewhat unreliable. Instead of writing the trap frame to
the top of the stack write it to the end of the heap instead. While this
could still corrupt some state it is hopefully still unused and will
definitely not overwrite previous return addresses. In the previous example
I now get a backtrace all the way up to main():

```
$ gdb -q test/test-fopen '-ex=b crt0.c:169' '-ex=target remote :1234' -ex=c -ex=bt
Reading symbols from test/test-fopen...
Breakpoint 1 at 0x800001c0: file ../../picolibc/picocrt/machine/riscv/crt0.c, line 168.
Remote debugging using :1234
0x0000000000001000 in ?? ()
Continuing.

Breakpoint 1, _trap () at ../../picolibc/picocrt/machine/riscv/crt0.c:169
169             __asm__("j      _ctrap");
#0  _trap () at ../../picolibc/picocrt/machine/riscv/crt0.c:168
#1  0x00000000800013d0 in fclose (f=0x804008c0) at ../../picolibc/newlib/libc/tinystdio/fclose.c:39
#2  0x0000000080000680 in main () at ../../picolibc/test/test-fopen.c:85
```

Author: arichardson

picocrt/machine/riscv/crt0.c

@@ -104,7 +104,7 @@ _trap(void)
 	__asm__(".option	push\n"
                 ".option	norelax\n"
                 "csrrw  sp, mscratch, sp\n"
-                "la	sp, __stack\n"
+                "la	sp, __heap_end\n"
                 ".option	pop");
 
         /* Make space for saved registers */