exec: Do not use absolute address hints for code_gen_buffer with -fpie

The hard-coded addresses inside alloc_code_gen_buffer only make sense
if we're building an executable that will actually run at the address
we've put into the linker scripts.

When we're building with -fpie, the executable will run at some
random location chosen by the kernel.  We get better placement for
the code_gen_buffer if we allow the kernel to place the memory,
as it will tend to to place it near the executable, based on the
PROT_EXEC bit.

Since code_gen_prologue is always inside the executable, this effect
is easily seen at the end of most TB, with the exit_tb opcode, and
with any calls to helper functions.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

exec.c

@@ -578,7 +578,12 @@ static inline void *alloc_code_gen_buffer(void)
     /* Constrain the position of the buffer based on the host cpu.
        Note that these addresses are chosen in concert with the
        addresses assigned in the relevant linker script file.  */
-# if defined(__x86_64__) && defined(MAP_32BIT)
+# if defined(__PIE__) || defined(__PIC__)
+    /* Don't bother setting a preferred location if we're building
+       a position-independent executable.  We're more likely to get
+       an address near the main executable if we let the kernel
+       choose the address.  */
+# elif defined(__x86_64__) && defined(MAP_32BIT)
     /* Force the memory down into low memory with the executable.
        Leave the choice of exact location with the kernel.  */
     flags |= MAP_32BIT;