[gdb/symtab] Work around PR gas/29517

When using glibc debuginfo generated with gas 2.39, we run into PR gas/29517:
...
$ gdb -q -batch a.out -ex start -ex "p (char *)strstr (\"haha\", \"ah\")"
Temporary breakpoint 1 at 0x40051b: file hello.c, line 6.

Temporary breakpoint 1, main () at hello.c:6
6	  printf ("hello\n");
Invalid cast.
...
while without glibc debuginfo installed we get the expected result:
...
$n = 0x7ffff7daa1b1 "aha"
...
and likewise with glibc debuginfo generated with gas 2.40.

The strstr ifunc resolves to __strstr_sse2_unaligned.  The problem is that gas
generates dwarf that states that the return type is void:
...
<1><3e1e58>: Abbrev Number: 2 (DW_TAG_subprogram)
    <3e1e59>   DW_AT_name        : __strstr_sse2_unaligned
    <3e1e5d>   DW_AT_external    : 1
    <3e1e5e>   DW_AT_low_pc      : 0xbbd2e
    <3e1e66>   DW_AT_high_pc     : 0xbc1c3
...
while the return type should be a DW_TAG_unspecified_type, as is the case
with gas 2.40.

We can still use the workaround of casting to another function type for both
__strstr_sse2_unaligned:
...
(gdb) p ((char * (*) (const char *, const char *))__strstr_sse2_unaligned) \
  ("haha", "ah")
$n = 0x7ffff7daa211 "aha"
...
and strstr (which requires using *strstr to dereference the ifunc before we
cast):
...
gdb) p ((char * (*) (const char *, const char *))*strstr) ("haha", "ah")
$n = 0x7ffff7daa251 "aha"
...
but that's a bit cumbersome to use.

Work around this in the dwarf reader, such that we have instead:
...
(gdb) p (char *)strstr ("haha", "ah")
$n = 0x7ffff7daa1b1 "aha"
...

This also requires fixing producer_is_gcc to stop returning true for
producer "GNU AS 2.39.0".

Tested on x86_64-linux.

Approved-By: Andrew Burgess <aburgess@redhat.com>

PR symtab/30911
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30911

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