travis: reduce coverage of gprof build

This build is regularly timing out and even switching off linux-user
wasn't enough. Instead explicitly choose a target list of broadly the
"major" architectures. This is enough to check the gprof build
machinery works without worrying about the actual coverage results.

I did try various YAML constructs for specifying CONFIG with
continuation but couldn't get any of them to work hence the very long
line.
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>

.travis.yml

@@ -80,7 +80,7 @@ matrix:
     - env: CONFIG="--disable-user"
       compiler: clang
     # gprof/gcov are GCC features
-    - env: CONFIG="--enable-gprof --enable-gcov --disable-pie --disable-linux-user"
+    - env: CONFIG="--enable-gprof --enable-gcov --disable-pie --target-list=aarch64-softmmu,arm-softmmu,i386-softmmu,mips-softmmu,mips64-softmmu,ppc64-softmmu,riscv64-softmmu,s390x-softmmu,x86_64-softmmu"
       compiler: gcc
     # We manually include builds which we disable "make check" for
     - env: CONFIG="--enable-debug --enable-tcg-interpreter"