libllvm: make clang++ order -lm after -lgcc

Without this patch, libm will be linked before libgcc (which is a linker script that links to both libunwind and libgcc_real); as libm.so in our system exports unwind symbols, it prevents the built objects from being linked to the unwinders properly.

Since libm is a part of bionic, we can't ship it like we do for libc++_shared or zlib. Therefore, patching clang++ is probably the only way.

packages/libllvm/build.sh

@@ -2,7 +2,7 @@ TERMUX_PKG_HOMEPAGE=https://clang.llvm.org/
 TERMUX_PKG_DESCRIPTION="Modular compiler and toolchain technologies library"
 TERMUX_PKG_LICENSE="NCSA"
 TERMUX_PKG_VERSION=8.0.0
-TERMUX_PKG_REVISION=3
+TERMUX_PKG_REVISION=4
 TERMUX_PKG_SHA256=(8872be1b12c61450cacc82b3d153eab02be2546ef34fa3580ed14137bb26224c
                    084c115aab0084e63b23eee8c233abb6739c399e29966eaeccfc6e088e0b736b
 		   9caec8ec922e32ffa130f0fb08e4c5a242d7e68ce757631e425e9eba2e1a6e37

packages/libllvm/cxx-libm.patch

@@ -0,0 +1,22 @@
+diff --git a/tools/clang/lib/Driver/ToolChains/Gnu.cpp~ b/tools/clang/lib/Driver/ToolChains/Gnu.cpp
+index 2ad4509..4d0b060 100644
+--- a/tools/clang/lib/Driver/ToolChains/Gnu.cpp~
++++ b/tools/clang/lib/Driver/ToolChains/Gnu.cpp
+@@ -483,7 +483,6 @@ void tools::gnutools::Linker::ConstructJob(Compilation &C, const JobAction &JA,
+       if (OnlyLibstdcxxStatic)
+         CmdArgs.push_back("-Bdynamic");
+     }
+-    CmdArgs.push_back("-lm");
+   }
+   // Silence warnings when linking C code with a C++ '-stdlib' argument.
+   Args.ClaimAllArgs(options::OPT_stdlib_EQ);
+@@ -513,6 +512,9 @@ void tools::gnutools::Linker::ConstructJob(Compilation &C, const JobAction &JA,
+ 
+       AddRunTimeLibs(ToolChain, D, CmdArgs, Args);
+ 
++      if (D.CCCIsCXX())
++        CmdArgs.push_back("-lm");
++
+       if (WantPthread && !isAndroid)
+         CmdArgs.push_back("-lpthread");
+