tests: Allow GResource external data tests to use llvm-objcopy

Relax the requirement for the test to only be compiled/run under gcc,
since a version of LLVM was released which supports `--add-symbol`.

`objcopy` should be overrideable to be `llvm-objcopy` by using a machine
file as per https://mesonbuild.com/Machine-files.html#binaries.

Suggested and tested by Grigory Vasilyev.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Fixes: #2423
10 jobs for 2423-llvm-objcopy in 25 minutes and 37 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Style Check
passed #1346349
check-todos

00:01:57

passed #1346348
style-check-diff

00:02:05

 
  Build
passed #1346352
cross-android_api21_arm64

00:01:48

passed #1346353
cross-android_api28_arm64

00:02:24

passed #1346354
cross-mingw64

00:03:16

passed #1346351
debian-stable-x86_64

00:03:19

passed #1346350
fedora-x86_64

00:09:09

passed #1346355
win32-ps
msys2-mingw32

00:15:38

passed #1346356
win32-ps
vs2017-x64

00:09:18

 
  Coverage
passed #1346357
coverage

00:02:03

76.2%