Update the compiler-builtins submodule

Pull in a fix for japaric/libm#129

diff --git a/src/libcompiler_builtins b/src/libcompiler_builtins
index b4a3645cfe102a100b6ebecbe76df37bc957eda8..52a6a4d7087d14a35d44a11c39c77fa79d71378d 160000 (submodule)
--- a/src/libcompiler_builtins
+++ b/src/libcompiler_builtins
@@ -1 +1 @@
-Subproject commit b4a3645cfe102a100b6ebecbe76df37bc957eda8
+Subproject commit 52a6a4d7087d14a35d44a11c39c77fa79d71378d