Rename target.linux[_x86[_64]] to target.linux_glibc[_x86[_64]]

In the future, target.linux will apply to all targets running a linux kernel
(android, linux_glibc, linux_bionic). So move all current users to the specific
linux_glibc.

There will be another cleanup pass later that will move some instances back to
target.linux if the properties should be shared with target.android and
target.linux_bionic, but target.linux needs to be removed first.

Test: out/soong/build.ninja identical before/after
Change-Id: Iadc1ba324e03fd2b1b52bf471ffa2a71e0ec3e61
Exempt-From-Owner-Approval: build system cleanup
diff --git a/btcore/Android.bp b/btcore/Android.bp
index b91091c..eba1ae3 100644
--- a/btcore/Android.bp
+++ b/btcore/Android.bp
@@ -20,7 +20,7 @@
         darwin: {
             enabled: false,
         },
-        linux: {
+        linux_glibc: {
             cflags: ["-D_GNU_SOURCE"],
         },
     },