tcg: Remove softmmu code_gen_buffer fixed address

The commentary talks about "in concert with the addresses
assigned in the relevant linker script", except there is no
linker script for softmmu, nor has there been for some time.

(Do not confuse the user-only linker script editing that was
removed in the previous patch, because user-only does not
use this code_gen_buffer allocation method.)
Reviewed-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NThomas Huth <thuth@redhat.com>
Signed-off-by: NRichard Henderson <richard.henderson@linaro.org>

accel/tcg/translate-all.c

@@ -1043,47 +1043,20 @@ static inline void *alloc_code_gen_buffer(void)
 {
     int prot = PROT_WRITE | PROT_READ | PROT_EXEC;
     int flags = MAP_PRIVATE | MAP_ANONYMOUS;
-    uintptr_t start = 0;
     size_t size = tcg_ctx->code_gen_buffer_size;
     void *buf;
 
-    /* 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(__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;
-    /* Cannot expect to map more than 800MB in low memory.  */
-    if (size > 800u * 1024 * 1024) {
-        tcg_ctx->code_gen_buffer_size = size = 800u * 1024 * 1024;
-    }
-# elif defined(__sparc__)
-    start = 0x40000000ul;
-# elif defined(__s390x__)
-    start = 0x90000000ul;
-# elif defined(__mips__)
-#  if _MIPS_SIM == _ABI64
-    start = 0x128000000ul;
-#  else
-    start = 0x08000000ul;
-#  endif
-# endif
-
-    buf = mmap((void *)start, size, prot, flags, -1, 0);
+    buf = mmap(NULL, size, prot, flags, -1, 0);
     if (buf == MAP_FAILED) {
         return NULL;
     }
 
 #ifdef __mips__
     if (cross_256mb(buf, size)) {
-        /* Try again, with the original still mapped, to avoid re-acquiring
-           that 256mb crossing.  This time don't specify an address.  */
+        /*
+         * Try again, with the original still mapped, to avoid re-acquiring
+         * the same 256mb crossing.
+         */
         size_t size2;
         void *buf2 = mmap(NULL, size, prot, flags, -1, 0);
         switch ((int)(buf2 != MAP_FAILED)) {