hifive1-qemu: increase memory to 64K

Somehow this is accepted by QEMU. I'm doing this so that tests for
-target=hifive1-qemu still work with the RISC-V tasks scheduler (with a
stack size of 2048 bytes).

targets/hifive1-qemu.ld

@@ -1,11 +1,14 @@
 
 /* memory map:
  * https://github.com/sifive/freedom-e-sdk/blob/v201908-branch/bsp/sifive-hifive1/metal.default.lds
+ * With one change: this linkerscript uses 64K RAM instead of 16K as specified
+ * in metal.default.lds. Not sure why this works, but it works, and it avoids
+ * out-of-memory issues when running tests.
  */
 MEMORY
 {
     FLASH_TEXT (rw) : ORIGIN = 0x20400000, LENGTH = 0x1fc00000
-    RAM (xrw)       : ORIGIN = 0x80000000, LENGTH = 0x4000
+    RAM (xrw)       : ORIGIN = 0x80000000, LENGTH = 64K
 }
 
 _stack_size = 2K;