[gdb/symtab] Workaround PR gas/31115

On arm-linux, with gas 2.40, I run into:
...
(gdb) x /i main+8^M
   0x4e1 <main+7>:      vrhadd.u16      d14, d14, d31^M
(gdb) FAIL: gdb.arch/pr25124.exp: disassemble thumb instruction (1st try)
...

This is a regression due to PR gas/31115, which makes gas produce a low_pc
with the thumb bit set (0x4d8 & 0x1):
...
 <1><24>: Abbrev Number: 2 (DW_TAG_subprogram)
    <25>   DW_AT_name        : main
    <29>   DW_AT_external    : 1
    <29>   DW_AT_type        : <0x2f>
    <2a>   DW_AT_low_pc      : 0x4d9
    <2e>   DW_AT_high_pc     : 12
...

The regression was introduced in 2.39, and is also present in 2.40 and 2.41,
and hasn't been fixed yet.

Work around this in read_func_scope, by using gdbarch_addr_bits_remove on
low_pc and high_pc.

Tested on arm-linux and x86_64-linux.

PR tdep/31453
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31453