rustc marks .debug_gdb_scripts as ALLOC, LOAD

[whitequark]

To reproduce:

echo "fn main(){}" >t.rs
rustc -g t.rs
objdump -h t|grep -A1 debug_gdb_scripts

 16 .debug_gdb_scripts 00000022  000000000003ddc1  000000000003ddc1  0003ddc1  2**0
                  CONTENTS, ALLOC, LOAD, READONLY, DATA

This is not a big deal on PCs, but somewhat more important on a microcontroller. On top of that, it produces extremely surprising messages when flashing the firmware via gdb (which is how I discovered it):

Loading section .text, size 0x1ae lma 0x0
Loading section .debug_gdb_scripts, size 0x22 lma 0x1ae

... and the debug script path potentially ends in the non-volatile storage, unnecessarily exposing information about the build environment.

[whitequark]

cc @japaric

[whitequark]

Compare this to the output of clang translating a GDB example:

/* Note: The "MS" section flags are to remove duplicates.  */
#define DEFINE_GDB_PY_SCRIPT(script_name) \
  asm("\
.pushsection \".debug_gdb_scripts\", \"MS\",@progbits,1\n\
.byte 1 /* Python */\n\
.asciz \"" script_name "\"\n\
.popsection \n\
");

DEFINE_GDB_PY_SCRIPT("foo.py")

clang -c t.c
objdump -h t.o|grep -A1 debug_gdb_scripts

  1 .debug_gdb_scripts 00000008  0000000000000000  0000000000000000  00000040  2**0
                  CONTENTS, READONLY, DEBUGGING

[whitequark]

Closing as per discussion in #41627 (comment).