Fix issue 18014 - dmd test suite fails to link (part 1/2)

... on Linux x86_64 distros where PIC/PIE is enforced.

For more details, see:
* https://wiki.gentoo.org/wiki/Hardened/Position_Independent_Code_internals
* https://fedoraproject.org/wiki/Packaging:Guidelines#PIE
* https://wiki.debian.org/Hardening/PIEByDefaultTransition
* https://wiki.ubuntu.com/SecurityTeam/PIE
* non-PIE linker support removed in Android 5.0:
https://source.android.com/security/enhancements/enhancements50
* https://eli.thegreenplace.net/2011/11/03/position-independent-code-pic-in-shared-libraries/

We addressed the issues in the `dmd.conf` generated when dmd is build, the
`dmd.conf` files we ship in the release packages, and the makefiles of
druntime and phobos, but we haven't (yet) fixed the test suite build
system. Because of that, on Linux x86_64 distros where PIC is enforced,
the dmd test suite (the d_do_test.d test runner, as well as all test
cases that require linking) fails to link and as a result is completely
unusable.

The first commit of this pull-request does the following:
* adds a `PIC` variable to `test/Makefile`, which defaults to `1`, on x86_64
* exports a `PIC_FLAG` environment variable, which is set to `-fPIC` when
`PIC` is set to `1`
* adds `$(PIC_FLAG)` to `d_do_test.d`'s build command-line
* appends `$(PIC_FLAG)` to the `REQUIRED_ARGS` variable, which is
expanded in each `*.d` test case command-line

The second and final commit adds the `$PIC_FLAG` environment variable to the
dmd command-line in the shell-driven test cases, where the object files were
linked. This was done by trial and error, by amending each failing test case,
until I got the whole test suite to pass on my Ubuntu 17.10 system, minus two
unrelated test failures:
* `runnable/test_cdvecfill.d` - see
https://issues.dlang.org/show_bug.cgi?id=18013
* runnable/test17559.d - stack traces don't work properly on my system,
but I have yet to investigate the root cause.

See also:
* #7002

* dlang/druntime#1880
* dlang/druntime#1974

* dlang/phobos#5586
* dlang/phobos#5868

test/Makefile

@@ -106,6 +106,8 @@ export OBJ=.obj
 export DSEP=\\
 export SEP=$(subst /,\,/)
 
+PIC?=0
+
 DRUNTIME_PATH=..\..\druntime
 PHOBOS_PATH=..\..\phobos
 export DFLAGS=-I$(DRUNTIME_PATH)\import -I$(PHOBOS_PATH)
@@ -137,6 +139,21 @@ DMD_MODEL=64
 endif
 export DMD=../generated/$(OS)/$(BUILD)/$(DMD_MODEL)/dmd
 
+# default to PIC on x86_64, use PIC=1/0 to en-/disable PIC.
+# Note that shared libraries and C files are always compiled with PIC.
+ifeq ($(PIC),)
+    ifeq ($(MODEL),64) # x86_64
+        PIC:=1
+    else
+        PIC:=0
+    endif
+endif
+ifeq ($(PIC),1)
+    export PIC_FLAG:=-fPIC
+else
+    export PIC_FLAG:=
+endif
+
 DRUNTIME_PATH=../../druntime
 PHOBOS_PATH=../../phobos
 # link against shared libraries (defaults to true on supported platforms, can be overridden w/ make SHARED=0)
@@ -154,6 +171,8 @@ export D_OBJC=1
 endif
 endif
 
+DEBUG_FLAGS=$(PIC_FLAG) -g
+
 export DMD_TEST_COVERAGE=
 
 runnable_tests=$(wildcard runnable/*.d) $(wildcard runnable/*.sh)
@@ -224,7 +243,9 @@ start_fail_compilation_tests: $(RESULTS_DIR)/.created $(RESULTS_DIR)/d_do_test$(
 
 $(RESULTS_DIR)/d_do_test$(EXE): d_do_test.d $(RESULTS_DIR)/.created
 	@echo "Building d_do_test tool"
-	@echo "OS: $(OS)"
-	$(QUIET)$(DMD) -conf= $(MODEL_FLAG) -unittest -run d_do_test.d -unittest
-	$(QUIET)$(DMD) -conf= $(MODEL_FLAG) -od$(RESULTS_DIR) -of$(RESULTS_DIR)$(DSEP)d_do_test$(EXE) d_do_test.d
+	@echo "OS: '$(OS)'"
+	@echo "MODEL: '$(MODEL)'"
+	@echo "PIC: '$(PIC_FLAG)'"
+	$(DMD) -conf= $(MODEL_FLAG) $(DEBUG_FLAGS) -unittest -run d_do_test.d -unittest
+	$(DMD) -conf= $(MODEL_FLAG) $(DEBUG_FLAGS) -od$(RESULTS_DIR) -of$(RESULTS_DIR)$(DSEP)d_do_test$(EXE) d_do_test.d