LAKITU: Set EGIT_MASTER=main for some llvm packages

The master branch has been removed from github.com/llvm/llvm-project.
This breaks git-2.eclass. We need to set EGIT_MASTER=main now for these
packages.

I'm making this patch to fix the build. I will try to see how ChromeOS
is resolving this after the build is fixed.

BUG=b/178727618
TEST=emerge-lakitu --root-deps=rdeps --select --quiet sys-libs/libcxx
RELEASE_NOTE=None

cos-patch: bug
Reviewed-on: https://cos-review.googlesource.com/c/third_party/overlays/chromiumos-overlay/+/11510
Tested-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Reviewed-by: Dexter Rivera <riverade@google.com>
Change-Id: I201bdeb042f96fea79e6434041bf2bbf579caad9
Reviewed-on: https://cos-review.googlesource.com/c/third_party/overlays/chromiumos-overlay/+/11570
Main-Branch-Verified: Robert Kolchmeyer <rkolchmeyer@google.com>
diff --git a/chromeos/config/env/sys-devel/llvm b/chromeos/config/env/sys-devel/llvm
index c8ca095..f3f477a 100644
--- a/chromeos/config/env/sys-devel/llvm
+++ b/chromeos/config/env/sys-devel/llvm
@@ -6,3 +6,5 @@
 cros_pre_src_prepare_filter_sanitizers() {
 	tc-is-clang && append-flags "-fno-sanitize=vptr"
 }
+
+EGIT_MASTER="main"
diff --git a/chromeos/config/env/sys-libs/libcxx b/chromeos/config/env/sys-libs/libcxx
index fc031b6..bbff30e 100644
--- a/chromeos/config/env/sys-libs/libcxx
+++ b/chromeos/config/env/sys-libs/libcxx
@@ -5,3 +5,5 @@
 cros_pre_src_prepare_enable_cxx_exceptions() {
       cros_enable_cxx_exceptions
 }
+
+EGIT_MASTER="main"
diff --git a/chromeos/config/env/sys-libs/libcxxabi b/chromeos/config/env/sys-libs/libcxxabi
index 4abc671..d8922e1 100644
--- a/chromeos/config/env/sys-libs/libcxxabi
+++ b/chromeos/config/env/sys-libs/libcxxabi
@@ -7,3 +7,5 @@
 cros_pre_src_prepare_enable_cxx_exceptions() {
 	cros_enable_cxx_exceptions
 }
+
+EGIT_MASTER="main"