plan9: reduce the set of architecture-dependent files on Plan 9

From main repo: https://go-review.googlesource.com/#/c/2167

Change-Id: I2043458a432ccd2aa8b206adc107ac7b215e355f
Reviewed-on: https://go-review.googlesource.com/8720
Reviewed-by: Rob Pike <r@golang.org>

plan9/const_plan9.go

@@ -0,0 +1,59 @@
+package plan9
+
+// Plan 9 Constants
+
+// Open modes
+const (
+	O_RDONLY  = 0
+	O_WRONLY  = 1
+	O_RDWR    = 2
+	O_TRUNC   = 16
+	O_CLOEXEC = 32
+	O_EXCL    = 0x1000
+)
+
+// Rfork flags
+const (
+	RFNAMEG  = 1 << 0
+	RFENVG   = 1 << 1
+	RFFDG    = 1 << 2
+	RFNOTEG  = 1 << 3
+	RFPROC   = 1 << 4
+	RFMEM    = 1 << 5
+	RFNOWAIT = 1 << 6
+	RFCNAMEG = 1 << 10
+	RFCENVG  = 1 << 11
+	RFCFDG   = 1 << 12
+	RFREND   = 1 << 13
+	RFNOMNT  = 1 << 14
+)
+
+// Qid.Type bits
+const (
+	QTDIR    = 0x80
+	QTAPPEND = 0x40
+	QTEXCL   = 0x20
+	QTMOUNT  = 0x10
+	QTAUTH   = 0x08
+	QTTMP    = 0x04
+	QTFILE   = 0x00
+)
+
+// Dir.Mode bits
+const (
+	DMDIR    = 0x80000000
+	DMAPPEND = 0x40000000
+	DMEXCL   = 0x20000000
+	DMMOUNT  = 0x10000000
+	DMAUTH   = 0x08000000
+	DMTMP    = 0x04000000
+	DMREAD   = 0x4
+	DMWRITE  = 0x2
+	DMEXEC   = 0x1
+)
+
+const (
+	STATMAX    = 65535
+	ERRMAX     = 128
+	STATFIXLEN = 49
+)