arm: use armv7 instead of thumbv7

At the moment, thumbv7 is crashing. I'm not exactly sure why, but it
appears that there is an unknown instruction in __aeabi_uldivmod
(probably from libgcc).

I've fixed this by switching to armv7, which is also somewhat modern.
Maybe we can switch back to Thumb2 (aka thumbv7) once we start using
musl and compiler-rt. In the meantime, this does fix a miscompilation
(illegal instruction).

compileopts/target.go

@@ -172,7 +172,7 @@ func LoadTarget(target string) (*TargetSpec, error) {
 			"386":   "i386",
 			"amd64": "x86_64",
 			"arm64": "aarch64",
-			"arm":   "thumbv7",
+			"arm":   "armv7",
 		}[goarch]
 		if llvmarch == "" {
 			llvmarch = goarch