gh-35114: libgap: Remove some GC hazards

    
### 📚 Description

Trac branch `u/gh-collares/gap-gc` from #34701, now migrated to GitHub.
Currently based atop #35093; will rebase once that is merged.

The rest of the description below is copied from #34701:

A refactor in #27946 introduced "unprotected" (not surrounded by
`GAP_Enter`/`GAP_Leave`) `GAP_ValueGlobalVariable` calls. I believe this
might be a GC hazard, because after updating to GAP 4.12.1 I started
seeing aarch64 crashes on NixOS infrastructure such as:

```
#0  0x0000fffff79740e8 in wait4 ()
#1  0x0000fffff5dc6b78 in print_enhanced_backtrace ()
#2  0x0000fffff5dc8190 in sigdie ()
#3  0x0000fffff5dcb1c0 in cysigs_signal_handler ()
#4  0x0000fffff7ffb7cc in __kernel_rt_sigreturn ()
#5  0x0000ffff99a0bf28 in ConvString ()
#6  0x0000000000000000 in ?? ()
#7  0x0000000000000000 in ?? ()
#8  0x0000000000000000 in ?? ()
#9  0x0000ffff99989930 in Pr ()
#10 0x0000ffff9998aa18 in CloseOutput ()
#11 0x0000ffff99884828 in capture_stdout () at /build/sage-
src-9.7/pkgs/sagemath-standard/sage/libs/gap/element.pyx:154
...
```
I also see cases where `capture_stdout` throws errors such as
`sage.libs.gap.util.GAPError: Error, Length: <list> must be a list (not
the integer 255)` and then crashes. Both types of errors are fixed by
this ticket.

Note that I am nesting `GAP_Enter`/`GAP_Leave` calls because I didn't
remove the preexisting calls inside `capture_stdout`. That's because I
feared removing the innermost calls might create a new footgun (and I
believe nested `GAP_Enter`/`GAP_Leave` calls are explicitly supported),
but removing them should cause no problem. Removing them might even be
preferable for performance reasons, I don't know.

Fixes #34701

### 📝 Checklist

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### ⌛ Dependencies
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- #35093: GAP 4.12.2 upgrade, which touches the same function and should
land first.
    
URL: #35114
Reported by: Mauricio Collares
Reviewer(s): Dima Pasechnik

src/sage/libs/gap/element.pyx

@@ -166,9 +166,13 @@ cdef char *gap_element_repr(Obj obj):
     GAP on the command-line (i.e. when evaluating an expression that returns
     that object.
     """
-
-    cdef Obj func = GAP_ValueGlobalVariable("ViewObj")
-    return capture_stdout(func, obj)
+    cdef Obj func
+    try:
+        GAP_Enter()
+        func = GAP_ValueGlobalVariable("ViewObj")
+        return capture_stdout(func, obj)
+    finally:
+        GAP_Leave()
 
 
 cdef char *gap_element_str(Obj obj):
@@ -181,8 +185,13 @@ cdef char *gap_element_str(Obj obj):
     slightly different approach more closely mirroring Python's str/repr
     difference (though this does not map perfectly onto GAP).
     """
-    cdef Obj func = GAP_ValueGlobalVariable("Print")
-    return capture_stdout(func, obj)
+    cdef Obj func
+    try:
+        GAP_Enter()
+        func = GAP_ValueGlobalVariable("Print")
+        return capture_stdout(func, obj)
+    finally:
+        GAP_Leave()
 
 
 cdef Obj make_gap_record(sage_dict) except NULL: