config: swap to glibc 2.17

**Please do not revert this change without consulting with either
srhines@ or gbiv@ first. It's a large change, and if it only causes
simple/small breakages, we plan to fix those as they pop up.**

This CL swaps the host Android Linux toolchain to gcc 4.8.3 + glibc
2.17, landed in Ieeed444801160e3d288bbad7d871067e81411118.

Bug: 111358231
Test: We've done extensive testing on internal master. Please see
      Ieeed444801160e3d288bbad7d871067e81411118.
Change-Id: If991a603a3dce33b55086672c27a11992f0f3123
diff --git a/cc/config/x86_linux_host.go b/cc/config/x86_linux_host.go
index 8da21b3..f072f34 100644
--- a/cc/config/x86_linux_host.go
+++ b/cc/config/x86_linux_host.go
@@ -111,14 +111,22 @@
 )
 
 const (
-	linuxGccVersion = "4.8"
+	linuxGccVersion   = "4.8.3"
+	linuxGlibcVersion = "2.17"
 )
 
 func init() {
 	pctx.StaticVariable("LinuxGccVersion", linuxGccVersion)
+	pctx.StaticVariable("LinuxGlibcVersion", linuxGlibcVersion)
+	// Most places use the full GCC version. A few only use up to the first two numbers.
+	if p := strings.Split(linuxGccVersion, "."); len(p) > 2 {
+		pctx.StaticVariable("ShortLinuxGccVersion", strings.Join(p[:2], "."))
+	} else {
+		pctx.StaticVariable("ShortLinuxGccVersion", linuxGccVersion)
+	}
 
 	pctx.SourcePathVariable("LinuxGccRoot",
-		"prebuilts/gcc/${HostPrebuiltTag}/host/x86_64-linux-glibc2.15-${LinuxGccVersion}")
+		"prebuilts/gcc/${HostPrebuiltTag}/host/x86_64-linux-glibc${LinuxGlibcVersion}-${ShortLinuxGccVersion}")
 
 	pctx.StaticVariable("LinuxGccTriple", "x86_64-linux")