Use require is_amd64_regs_target

This changes some tests to use "require is_amd64_regs_target".

gdb/testsuite/gdb.arch/amd64-gs_base.exp

@@ -15,10 +15,7 @@
 
 standard_testfile
 
-if {![is_amd64_regs_target]} {
-    verbose "Untested x86_64 fs_base and gs_base tests."
-    return
-}
+require is_amd64_regs_target
 
 if { [prepare_for_testing "failed to prepare" ${testfile} ${srcfile} \
      [list debug nowarnings]] } {

gdb/testsuite/gdb.arch/amd64-init-x87-values.exp

@@ -18,9 +18,7 @@
 # Test initial values of x87 control registers, before any x87
 # instructions have been executed in the inferior.
 
-if {![is_amd64_regs_target]} {
-    return
-}
+require is_amd64_regs_target
 
 standard_testfile .S
 

gdb/testsuite/gdb.base/disasm-optim.exp

@@ -15,9 +15,7 @@
 
 # This test exercises disassemble /s with optimized and inlined code.
 
-if { ![is_amd64_regs_target] } {
-    return
-}
+require is_amd64_regs_target
 
 standard_testfile .S
 

gdb/testsuite/gdb.trace/tracefile-pseudo-reg.exp

@@ -12,10 +12,7 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-if { ! [is_amd64_regs_target] } {
-    verbose "Skipping tfile AVX test (target is not x86_64)."
-    return
-}
+require is_amd64_regs_target
 
 load_lib "trace-support.exp"