[gdb/testsuite] Use prototype to call libc functions

[gdb/testsuite] Use prototype to call libc functions

On openSUSE Tumbleweed (using glibc 2.36), I run into:
...
(gdb) print /d (int) munmap (4198400, 4096)^M
Invalid cast.^M
(gdb) FAIL: gdb.base/break-main-file-remove-fail.exp: cmdline: \
  get integer valueof "(int) munmap (4198400, 4096)"
...

The problem is that after starting the executable, the symbol has type
"void (*) (void)":
...
(gdb) p munmap
$1 = {<text variable, no debug info>} 0x401030 <munmap@plt>
(gdb) start
  ...
(gdb) p munmap
$2 = {void (void)} 0x7ffff7feb9a0 <__GI_munmap>
...
which causes the "Invalid cast" error.

Looking at the debug info for glibc for symbol __GI_munmap:
...
 <0><189683>: Abbrev Number: 1 (DW_TAG_compile_unit)
    <189691>   DW_AT_name        : ../sysdeps/unix/syscall-template.S
    <189699>   DW_AT_producer    : GNU AS 2.39.0
<1><1896ae>: Abbrev Number: 2 (DW_TAG_subprogram)
    <1896af>   DW_AT_name        : __GI___munmap
    <1896b3>   DW_AT_external    : 1
    <1896b4>   DW_AT_low_pc      : 0x10cad0
    <1896bc>   DW_AT_high_pc     : 37
...
that's probably caused by this bit (or similar bits for other munmap aliases).

This is fixed in gas on trunk by commit 5578fbf672e ("GAS: Add a return type
tag to DWARF DIEs generated for function symbols").

Work around this (for say gas 2.39) by explicitly specifying the prototype for
munmap.

Likewise for getpid in a couple of other test-cases.

Tested on x86_64-linux.