libfastboot uses usb_linux.cpp for all Linux targets

The build system has added the new target named 'linux_cross' which is
the cross-compiled (i.e. arm on x86) host target. libfastboot is now
configured to use usb_linux.cpp not only for linux_glibc (which is the
native host target using glibc), but for all Linux-based host targets
including linux_glibc, linux_bionic, and the new linux_cross.

Note that the device target 'android' is also included in the 'linux'
target. But that doesn't cause a problem because libfastboot is a host
library which is not enabled for the device target.

Bug: 159685774
Test: HOST_CROSS_OS=linux_cross m
out/soong/host/linux_cross-arm64/bin/fastboot

Change-Id: I3a2191b0878a26914cb0282ecf41a45296827c04

fastboot/Android.bp

@@ -251,7 +251,7 @@ cc_library_host_static {
         darwin: {
             srcs: ["usb_osx.cpp"],
         },
-        linux_glibc: {
+        linux: {
             srcs: ["usb_linux.cpp"],
         },
     },