apply const trick to ptr_to_globals

This was missing in the previous attempt to fix it via [1]

This helps fix segfaults when compiling with clang ( seen on riscv64 )

[  452.428349] less[270]: unhandled signal 11 code 0x1 at 0x000000000000000c in busybox.nosuid[2ab7491000+ba000]
[  452.430246] CPU: 3 PID: 270 Comm: less Not tainted 5.15.13-yocto-standard #1
[  452.431323] Hardware name: riscv-virtio,qemu (DT)
[  452.431925] epc : 0000002ab74a19ee ra : 0000002ab74a19dc sp : 0000003fec6ec980
[  452.432725]  gp : 0000002ab754dcb0 tp : 0000003f88783800 t0 : 0000003f8878d4a0
[  452.433744]  t1 : 0000002ab749b00c t2 : 0000000000000000 s0 : 0000003fec6ecc38
[  452.434732]  s1 : 000000000000004c a0 : 00000000ffffffff a1 : 0000002ab754dde0
[  452.435861]  a2 : 0000000000000000 a3 : 0000000000000100 a4 : 0000002ab754f3a0
[  452.436787]  a5 : 0000002ab754f3a0 a6 : 0000000000000000 a7 : 0000002ab754f2a0
[  452.437974]  s2 : 0000000000000002 s3 : 0000002ab754b6c8 s4 : 0000002ab749b60e
[  452.438781]  s5 : 0000000000000000 s6 : 0000002ab754b6c8 s7 : 0000003f88943060
[  452.439723]  s8 : 0000003f88944050 s9 : 0000002ad8502e88 s10: 0000002ad8502de8
[  452.440538]  s11: 0000000000000014 t3 : 0000003f887fceb6 t4 : 0000003f8893af0c
[  452.441438]  t5 : 0000000000000000 t6 : 0000003f88923000

[1] https://git.busybox.net/busybox/commit/?id=1f925038a

Signed-off-by: Khem Raj <[email protected]>
Signed-off-by: Denys Vlasenko <[email protected]>

include/libbb.h

@@ -2292,7 +2292,7 @@ struct globals;
 /* '*const' ptr makes gcc optimize code much better.
  * Magic prevents ptr_to_globals from going into rodata.
  * If you want to assign a value, use SET_PTR_TO_GLOBALS(x) */
-extern struct globals *const ptr_to_globals;
+extern struct globals *BB_GLOBAL_CONST ptr_to_globals;
 
 #define barrier() asm volatile ("":::"memory")