Move target check into allow_altivec_tests

Pedro pointed out that only PPC can possibly have altivec, so we can
move the target check into allow_altivec_tests.

Author: tromey

gdb/testsuite/gdb.arch/altivec-abi.exp

@@ -22,7 +22,6 @@
 #
 
 require allow_altivec_tests
-require {istarget "powerpc*"}
 
 standard_testfile
 

gdb/testsuite/gdb.arch/altivec-regs.exp

@@ -22,7 +22,6 @@
 #
 
 require allow_altivec_tests
-require {istarget "powerpc*"}
 
 standard_testfile
 

gdb/testsuite/gdb.arch/powerpc-vector-regs.exp

@@ -20,7 +20,7 @@
 # 0 to 31 in each of the 16 bytes of each corresponding register, and
 # we then check if gdb sees these same values.
 
-require allow_altivec_tests {[istarget "powerpc*"}
+require allow_altivec_tests
 
 standard_testfile
 

gdb/testsuite/lib/gdb.exp

@@ -3471,6 +3471,11 @@ gdb_caching_proc allow_altivec_tests {
 	return 0
     }
 
+    if {![istarget powerpc*]} {
+	verbose "$me: PPC target required, returning 0" 2
+	return 0
+    }
+
     # Make sure we have a compiler that understands altivec.
     if [test_compiler_info gcc*] {
         set compile_flags "additional_flags=-maltivec"