[gdb/testsuite] Handle missing gold linker in gdb.base/morestack.exp

[gdb/testsuite] Handle missing gold linker in gdb.base/morestack.exp

When running test-case gdb.base/morestack.exp without the gold linker
installed, we run into:
...
Running src/gdb/testsuite/gdb.base/morestack.exp ...
gdb compile failed, collect2: fatal error: cannot find 'ld'
compilation terminated.
FAIL: gdb.base/morestack.exp: continue

                === gdb Summary ===

nr of expected passes            1
nr of unexpected failures        1
nr of untested testcases         1
...

The test-case needs the gold linker to run correctly (as explained in commit
b8d38ee425 "testsuite: Fix false FAIL for gdb.base/morestack.exp"), but
only prefers it, and doesn't require it.

Fix this by requiring the gold linker in the test-case.  Furthermore, silence
the compilation error by introducing a caching proc have_fuse_ld_gold and
using it in this and other test-cases that use -fuse-ld=gold.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-07-13  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (have_fuse_ld_gold): New caching proc.
	* gdb.base/gcore-tls-pie.exp: Use have_fuse_ld_gold.
	* gdb.base/gold-gdb-index.exp: Same.
	* gdb.base/morestack.exp: Same.