Add LBT_USE_RTLD_DEEPBIND capability

[staticfloat]

Certain tools such as sanitizers don't like us loading libraries with
RTLD_DEEPBIND, so since we already have the workarounds in place for
systems that don't have RTLD_DEEPBIND at all, let's change these from
using compile-time constants to instead use a runtime switch that can be
overridden through setting the environment variable
LBT_USE_RTLD_DEEPBIND=0 before running our program.

[staticfloat]

@tkf can you try this out, regarding #47 ?

[staticfloat]

@giordano do you have any idea what's wrong with the drone setup?

[giordano]

No, the alpine image started failing a few weeks ago but I've never got around to look into it 😞

[tkf]

regarding #47

Thanks a lot! I can confirm that this branch works with

make debug  # let it fail
cp /PATH/TO/THIS/libblastrampoline/src/build/libblastrampoline.so usr/lib/libblastrampoline.so
LBT_USE_RTLD_DEEPBIND=0 make debug

---

To be concrete, $PWD/Make.user contains

TOOLCHAIN=$(TOOLCHAIN_WORKTREE)/usr/tools

# use our new toolchain
USECLANG=1
override CC=$(TOOLCHAIN)/clang
override CXX=$(TOOLCHAIN)/clang++
export ASAN_SYMBOLIZER_PATH=$(TOOLCHAIN)/llvm-symbolizer

USE_BINARYBUILDER_LLVM=1

override SANITIZE=1
override SANITIZE_ADDRESS=1

# make the GC use regular malloc/frees, which are hooked by ASAN
override WITH_GC_DEBUG_ENV=1

# default to a debug build for better line number reporting
override JULIA_BUILD_MODE=debug

# enable LLVM assertions (which also enables pass debug output)
#override LLVM_ASSERTIONS=1

# make ASAN consume less memory
export ASAN_OPTIONS=detect_leaks=0:fast_unwind_on_malloc=0:allow_user_segv_handler=1:malloc_context_size=2

JULIA_PRECOMPILE=1

and $TOOLCHAIN_WORKTREE/Make.user contains

USE_BINARYBUILDER_LLVM=1
BUILD_LLVM_CLANG=1

On $TOOLCHAIN_WORKTREE I ran make -C deps install-llvm install-clang install-llvm-tools to isntall enough binaries in $(TOOLCHAIN_WORKTREE)/usr/tools.