libmach: accept OS X binary generated by external linker

Fixes cpu subtype check when using external linker which sets the CPU_SUBTYPE_LIB64 bit (1<<31).
Fixes #6197.

R=golang-dev, minux.ma, rsc
CC=golang-dev
https://golang.org/cl/13248046

src/libmach/executable.c

@@ -1076,7 +1076,7 @@ machdotout(int fd, Fhdr *fp, ExecHdr *hp)
 			return 0;
 		}
 
-		if (mp->cpusubtype != MACH_CPU_SUBTYPE_X86) {
+		if (mp->cpusubtype != MACH_CPU_SUBTYPE_X86 && mp->cpusubtype != MACH_CPU_SUBTYPE_X86_64) {
 			werrstr("bad MACH cpu subtype - not amd64");
 			return 0;
 		}

src/libmach/macho.h

@@ -87,6 +87,7 @@ enum {
 	MACH_CPU_TYPE_X86_64 = (1<<24)|7,
 	MACH_CPU_TYPE_X86 = 7,
 	MACH_CPU_SUBTYPE_X86 = 3,
+	MACH_CPU_SUBTYPE_X86_64 = (1<<31)|3,
 	MACH_EXECUTABLE_TYPE = 2,
 	MACH_SEGMENT_32 = 1,	/* 32-bit mapped segment */
 	MACH_SEGMENT_64 = 0x19,	/* 64-bit mapped segment */